I see AB's PR division has gone into overdrive.
"Unique" because they have fulcrum fees? Fidelity has been doing this with many of its funds for a long time. "Several prominent managers have selectively used fulcrum fees: Putnam, Vanguard, Janus, and most recently AllianceBernstein (AB)".
https://www.bbh.com/en-us/insights/are-fulcrum-fees-the-future--24868
"According to Frank Caruso, chief investment officer of U.S. Growth Equities at AllianceBernstein, the FlexFee funds have the “lowest possible” minimum levels."
Not close. There is no lowest possible, since fees can go negative. From M*, 2011:

Bridgeway Pays Shareholders to Invest, for Now Bridgeway Aggressive Investors 1 BRAGX is expected to have a negative 0.51% expense ratio according the most recent prospectus. Meanwhile, Bridgeway Micro-Cap Limited BRMCX is expected to have a 0.00% expense ratio.
The negative and zero expense ratios are possible because Houston-based Bridgeway levies a performance fee on top of its management fee. The funds have performed so poorly relative to their benchmark indexes that the fund's manager, instead of its shareholders, has to put money into the fund or waive its management fee to compensate shareholders until performance improves.

FYI: (This is a follow-up article.)
Fund manager AlllianceBernstein has started what the average investor might hope for: A flexible set of fees for funds in which the company receives only a nominal annual fee unless a fund outperforms a benchmark index.
Regards,
Ted
https://www.marketwatch.com/story/this-unique-mutual-fund-charges-less-than-an-index-fund-if-it-underperforms-2018-04-05/print
M* Snapshot FFLYX:
http://www.morningstar.com/funds/XNAS/FFLYX/quote.html

FYI: On Evidence-Based Investing and How You Can Profit From It
Regards,
Ted
https://ofdollarsanddata.com/the-war-between-fear-and-evidence-9c020e595dc5

As I read the comments, I thought did you read David's commentary this month. If you did you have your answer. This month and his commentary for several months going back. It seems obvious that the prospects for 10 years to get back to even is a very good argument for why you leave a chunk money in Money markets and Short term Bonds.
Of course market timing is difficult but losing 50% is also very difficult. Many people who thought they would never sell after the 2009 sold out never got back in or have just recently got back in the market.

@Davidmoran,
HA. I don’t want to make it personal. Sorry if I got carried away. Defensive? Maybe.
Please know that I claim no expertise in financial affairs and don’t recommend my approach to others. I simply love following and discussing financial matters, along with science & astronomy, because in those disciplines a given input equals a given outcome. In other words, both disciplines rely on logic and provable facts. Compounding works. Buying low and selling high is demonstrably more profitable than the reverse. Management fees make a difference in the long-run, etc. etc. Contrast that type of intelligent commentary with most of the garbage that gets consumed daily in our media driven society. Thus reason to read the board and share ideas.
Dick Strong and some of his cohorts gave market timing a bad name back in the late 90s. (I dunno how he escaped prison.) And the fund companies than were more or less forced to tighten their regulations to prevent timing. Without going into detail, the practice (timing) can be profitable for a few smart (or shrewd) investors, but “dings” the fund returns for most so invested (a practice sometimes called skimming).
Anyway, my plan - scatterbrained though it is - was designed to keep me from shooting myself in the foot by trading frequently. Just about everybody here, including myself, seems to agree that frequent trading is detrimental to long term returns. That’s why 75% is essentially “locked away” in a diversified core portfolio. Except for annual distributions and rare rebalancing it’s hands off with that portion.
The 25% “Flexible” portion is a concession to my perceived need to be “hands on.” I can’t tell you whether the incremental adjustments to cash/equity holdings based on perceived market risk over the years have worked or not. My guess is it’s probably been a “draw.” I can say that in ‘98 when the tech-bubble burst, bringing down the whole market, and again in late ‘08 / early ‘09 when the last bear market ended I did have a sizable cash stash to put to work. It felt good anyway to be buying low. But maybe I’d been better off if I hadn’t carried the cash/equity equation over the preceding years.
I know your OP was “Why cash?” ... It’s highly liquid for one thing. It doesn’t pose the same downside risk as short selling does. It doesn’t carry the high expenses / fees that using various derivatives would. And most fund companies don’t put restrictions on your ability to move in and out of their cash / cash equivalency accounts - as they do with their other funds. As I said earlier, bonds pose some special risks in this low rate environment that might not ordinarily exist.

The article is no less accurate than it was in 2010 (except for minor details like TD Waterhouse).
It describes the standardized A, B, C, I (institutional) class shares well. Going into any other share classes is like diving into quicksand.
Z shares - it's probably correct that these tend to be legacy (and closed) no load shares. I own representatives of the only two families I'm aware of: Columbia (formerly Acorn Funds), and Franklin Templeton (formerly Michael Price's Mutual Series Funds). That's not to say there aren't other examples; if there are, they're likely quite obscure.
S shares - here we sink even faster into the muck. The only family I'm aware of that created legacy S shares (analogous to Z shares) was Scudder. The original no load family (dating back to 1928), it was acquired by Zurich in 1997, then rebranded Scudder Kemper, with loads added. Then in 2001, Deutsche Bank bought Scudder from Zurich.
Throughout all of this, Scudder had a line of funds branded AARP, which first got a name change in 2000 from AARP fund xxx to Scudder fund xxx, class AARP. Still noload and open, as I recall. Sometime subsequent to Deutsche's acquisition, these were renamed DWS fund xxx. "On July 14, 2006, Class AARP shares were converted into Class S shares." That's from one of the funds' 2009 prospectuses. From vague memory, the non-AARP branded investor (noload, retail) share class had previously been renamed Class S.
So at least that example of S class shares is similar to Z shares, as stated in the article. But wait, it gets worse. The only other example of S shares I'm aware of is Selected Shares. The S share class there aren't legacy shares.
In May 2004, Selected Shares decided to give larger investors a break - similar to what Vanguard did with Admiral Shares. Selected Shares named its original shares class S (with its 12b-1 fee) and created a class D (no 12b-1 fee, $10K min). So here, the S shares are the more expensive ones, and they're still open - that was the whole idea.
Schwab and Fidelity balked at this, and initially kicked Selected Shares out of their supermarkets. Schwab's policy for any fund is either not to offer it NTF, or to offer the cheapest retail share class NTF. Apparently they reached an accommodation, where Schwab would offer the S shares NTF, but would require $100K to buy D shares. In that way, Schwab could present the fiction that D shares weren't really retail, so it was selling the cheapest (i.e. class S) Selected Shares NTF. See, e.g. SLASX and SLADX.
https://www.kiplinger.com/article/investing/T041-C009-S001-a-secret-about-buying-funds-through-an-online-brok.html
D shares - notice that the only example given is PIMCO. There is also the Selected Shares D class, but that just complicates matters. There may be other D shares, (I'd mentioned Janus above), but like Z shares they're likely quite obscure.

>> Fido Money Market- "FZDXX" is yielding 1.69% at the moment.
Well, study this in some detail -- fn5, fn4 / minimum if applicable, performance details --- it has outperformed (somewhat) 3-month t-bills; 1y as of last week was 1.16%
https://fundresearch.fidelity.com/mutual-funds/summary/31617H805?type=o-SrchResults
http://fundresearch.fidelity.com/mutual-funds/fundfactsheet/31617H805

I prefer a money market to short-term bond funds because right now FZDXX is at 1.70%, and each week it will increase a few basis points as long as interest rates continue to go up. And for the near-term, that's the trend.
For those parking funds in Cash for a few weeks or a few months in the interest of market-timing or whatever the reasoning, MMkts are ideal. The odds of a negative return are almost nil (unlike short-term bond funds). I've compared to TRBUX, GILPX, MINT, etc. Money markets are finally giving a small return, and people still think they are yielding close to 0%. That is finally changing.
My next level up is a Floating Rate fund. I own SPFPX .
And then SEMPX (MBS).
Not everybody has the desire to be at 100% equities. After a long bull market, some of us will get cautious. The market is finally showing some volatility after years of smooth sailing thanks to Fed intervention. That intervention has been slowly fading away.
Now we have a "leader" starting a trade war. Perhaps some caution is warranted. But that's just my 2 cents.
I have been buying ITOT this week on dips. But I keep a solid cash stake on hand to buy many, many more ITOT lots this summer on continued dips. If that doesn't happen, I'll be stuck earning close to +2%. A gamble I am comfortable with.

I did a quick (maybe not so quick) screen of M*'s target date funds on my brokerage platform (run by Fidelity).
Unfiltered:
-945 Retirement Date Funds appeared in the first unfiltered screen. Most fund descriptors include a specific year in time (2010 - 2060).
My first screen tried to identify the high barrier to entry funds. There were 180 funds that required more than a $1M minimum initial investment (the highest being $15M). Many of these funds had such small AUM that they appear to be set up for a small handful of high net worth investors who are worried about their retirement. Wish I had their problem.
This group of funds had a variety of descriptors that seemed worth sharing:
SmartRetirement (JP Morgan)
Balance Risk Retire (Invesco)
One Choice (American Century)
Lifepath Dynamic or Index or Smart Beta (BlackRock)
RealPath (Pimco)
Target (Manning & Nappier & Wells Fargo)
Retirement (AllianzGI)
LifeSmart (Franklin)
Multi manager & Multi Index (JHancock)
I then filter for funds with an initial minimum of $2,500 or less:
-750 funds appeared
I Filtered for: No Fee / TF (this removed loaded funds - more than half of the above 750 carried loads)
-366 funds appeared
I Filter out High ER funds (set ER at 1.0% or lower):
-325 funds appeared
I Filter for low ER (set ER at .5% or lower):
-105 funds appeared
I Filter for lowest ER (set ER at .3% or lower):
-36 funds appeared from four companies:
Vanguard, Fidelity, Wells Fargo and Schwab
Returning to the 325 fund choice screen (funds with an ER of 1% or lower)...
I screened for both a high 3 yr Sharpe Ratio and a high 3 year Alpha:
-45 funds appeared, with the vast majority of these funds being American Funds.
-Also, M* consistently gave many of these American Funds 5* rating over 1,3,5 & 10 yr
-American Funds have many share classes options (R6, R5 & F2 seem the cheapest)
-Vanguard had 1 fund (Target Income = VTINX).
-Fidelity had 1 fund (Fidelity Freedom income = FFFAX)
Finally, I lowered the ER back down to .05% (to capture the lowest ER share classes):
-23 funds appeared (21 fund are managed by American Funds and their initial minimum is $250)

how quickly we leave the conditions set in the OP
not talking about having cash for equity purchases unless you are doing timing, which is a separate discussion
not talking about having cash for nearterm (~n years) needs or indeed emergency stash / buffer
if for sleep-at-night, cool, just realize that and say so

Gosh - Market timing? Don’t know. You can judge. As laid out in some detail in one of Puddenhead’s threads last November, I split my portfolio between a “Core” position (75%) which doesn’t change and a “Flex” position (25%) in which I attempt to correlate my exposure to equities & cash with my perception of market risk at the time. The normal cash range runs from 10% to 20%. Not a perfect system for sure. But that’s the plan I’ve followed for 22 years since retiring and it meets my humble needs. Other than cash or short term bonds, where else would one move to when valuations appear high?
Now - Does that make me a “market timer“? Don’t know. I’ll say that all of my fund companies have strict rules designed to prevent market timing. Some I’ve been with for 30 or more years. None has ever identified me as a market timer or abusive trader. But if I am a timer, where’s your problem? It’s not illegal, unethical or immoral as far as I know. And it strikes me odd that the term would be tossed out in a derogatory fashion on a board where the most popular thread each month is: What are you buying, selling or pondering? Seems like a case of The pot calling the kettle black.
Re: Sleeping well ... I think having a clearly thought out investment plan and adhering to the plan rigorously does go a long way in assuring a good night’s sleep.
Cheers!

>> Fido Money Market- "FZDXX" is yielding 1.69% at the moment.
Well, study this in some detail -- fn5, fn4 / minimum if applicable, performance details --- it has outperformed (somewhat) 3-month t-bills; 1y as of last week was 1.16%
https://fundresearch.fidelity.com/mutual-funds/summary/31617H805?type=o-SrchResults
http://fundresearch.fidelity.com/mutual-funds/fundfactsheet/31617H805

FWIW: First Republic Bank is currently running the following rates in N California:
Term / Minimum / Rate / APY
6 Year / $10,000+ / 2.96% / 3.00%
5 Year / $10,000+ / 2.76% / 2.80%
4 Year / $10,000+ / 2.32% / 2.35%
3 Year / $10,000+ / 2.23% / 2.25%
2 Year / $10,000+ / 1.98% / 2.00%
18 Month / $10,000+ / 1.29% / 1.30%
12 Month / $10,000+ / 0.70% / 0.70%
6 Month / $10,000+ / 0.50% / 0.50%
3 Month / $10,000+ / 0.25% / 0.25%
30 Day / $25,000+ / 0.15% / 0.15%
23 Month Special / $10,000+ / 2.47% / 2.50%
16 Month Special / $10,000+ / 1.98% / 2.00%
8 Month Special / $10,000+ / 1.49% / 1.50%
Can almost start building a reasonable ladder to at least keep close to inflation. (At least "official" inflation.)

FYI: The first target-date funds were created in 1994, but after nearly a quarter-century of action they don’t seem to be wowing the investing public. At least that’s the indication of a recent survey of retirees and pre-retirees conducted by Massachusetts Mutual Life Insurance Company.
Regards,
Ted
https://www.fa-mag.com/news/investors-greet-target-date-funds-with-collective-yawn-37952.html?print

FYI: Of all the wild, head-scratching moves in financial markets this year, there are few that have surprised investors quite as much as the rally in defaulted Puerto Rico bonds. “It just blows my mind,” says Matt Dalton, chief executive officer of Belle Haven Investments.
Regards,
Ted
https://www.fa-mag.com/news/stunned-investors-reap-95--gains-on-defaulted-puerto-rico-bonds-37943.html?print

Hello,
My global hybrid sleeve holds my global allocation funds and accounts for about 10% of my overall portfolio. The sleeve produces a good dividend yield at about 4.25%. The funds held are CAIBX, PMAIX & TIBAX. PMAIX is classified by M* as a 30% to 50% equity allocation fund but currently has 30% in foreign equity and 17% in domestic equity, 31% in bonds, 13% in other assets and 9% in cash. With this, I felt its best fit was in my global hybrid sleeve.
In addition, the sleeve has a turnover ratio of about 75%. So, the funds seem to be active in changing their holdings and their positioning as the global investment enviornment changes. Also, within my portfolio, the sleeve seems to be the most adaptive to the ever changing market conditions due to the wide spectrum of assets the fund managers can choose from for investment.
With this, there is indeed a fit for global allocation funds within my portfolio.
Old_Skeet

There are always going to be some funds that are NTF at one brokerage and either sold with a load/fee or not sold at all at another brokerage. For example, IALAX is NTF at Fidelity, not available at Schwab.
Fidelity listing for IALAX.
Schwab listing for IALAX. (Availability = "Redemptions only")

We don't happen to use a global allocation fund, but we do use a moderate allocation fund (50 to 70% equity) to do most of the heavy lifting for us at around 60% of our portfolio. Then we use a few other funds to fill in the gaps such as global smallcap. We also use a bond fund to keep our equity to fixed income ratio where we want it.
So, while not a GA fund, same principle.
T. Rowe Price has a decent GA fund as well in RPGAX.

Further thoughts on growth and income (to supplement retirement income):
When I chart a cash choice (Ultrashort bond fund) like TSYYX (TSDOX) (which has a 24 year history) with a favorite growth fund (you pick yours)...I charted TSYYX with PRMTX... I imagine these two funds working together as the growth and income ingredients in a portfolio. The roughest growth period for PRMTX was between March 2000 - March 2010. It is during this spans of time one needed to have enough income stored in a fund like TSYYX to make distributions (for income) and make it through the under performance that PRMTX was experiencing.
For a long term growth and income investor, TSYYX might also serve a the funding source to reallocate into your growth fund (PRMTX) opportunistically as it under performed during time periods. TSYYX might also serve as the recipient of your growth as it is reallocated out of growth into income, again periodically.
Reallocating (re-balancing) between these two funds is one way of capturing these opportunities. Having two funds...one for growth fund and one for income...provides a place for these opportunities grow, to be harvested, to be stored and to be re-deployed when the time is right.
@davidrmoran: So yes, timing matters...it means raising cash when growth outperforms and redeploying from cash when growth under performs. The timing method is called "rules based - periodic re-allocation or re-balancing." I'm calling cash (income fund) any investment that is highly uncorrelated to the growth fund (market).
Source:
https://investopedia.com/terms/r/rebalancing.asp
@hank -Your cash holding may be serving this same purpose as a fund like FCONX or TSDOX (TSYYX). I guess I mention this because many investors forget that reallocation help a portfolio harvest gains that can be used for income or to be redeployed when growth opportunities arise. Cash can serve that purpose as easily as a conservative fund.
@MikeM...I'm easily impressed! Not going argue over a cash choice that works for you.
24 year chart:

@hank, Cash gets a bad rap.
As you stated, if you need cash for income you are not forced to sell when your investments are under performing.
Also, some investors consider credit cards or even HELOC (on their homes) as a source of cash which often come with costs (fees and interest rates). Many HELOCs were "called in" by lenders during the 2008 recession.
When liquidity tightens it is cash that provides opportunities and peace of mind.
Aside for true cash, how else do you hold the "cash" part of your portfolio?
I have been impressed with FCONX, RUSIX or TSYYX. Small, but steady returns:

Well now - “Double Dipping“ can also be a good thing. Think ice cream cones.
Truth is I have no idea what the future holds, nor does anyone else. Rather than trying to fathom dips, I’m just slogging through the mud as usual. Agree with the many smart folks who think valuations are stretched and have been for some time now (not a universal belief). That doesn’t mean you shouldn’t invest or that a crash is imminent. @Flack had an excellent post during the past few days about the difference in risk level that can be safely assumed by young workers who are averaging in over time and the risk appropriate for those already retired and having much shorter time horizons. I’d urge everyone to read that post and think about it - if they haven’t already done so.
Personally I had moved my “naked” cash position to above 22% around the first of the year. That’s the highest ever, and doesn’t even take into account the bond/fixed income held within my conservative allocation funds. As markets have pulled back maybe 5-10% over the past 3 weeks I’ve been able to put a little bit of that cash back to work - but it still sits unusually high at 20%+. In this I’m in agreement with several others who have recently referenced higher than normal cash positions.
As I hope I made clear, it’s the valuations that I try to hew to. Try and forget the other garbage (macro-politics). It will drive you nuts and maybe even result in some bad decisions.

More than you may possibly want to know about AMZA.
https://seekingalpha.com/article/4158648-etf-month-6-amza-part-1