As Hank wrote, salary reduction is a pre-tax contribution to an employer-sponsored retirement plan (401(k), 403(b), SIMPLE IRA, etc.) that allows you to
defer income. In contrast, salary deduction does not reduce your current income.
A salary deduction is a contribution
to the retirement plan made by having the employer "deduct" the money from your paycheck. What isn't clear from the question is whether the employer is treating this as a "classic" (pre-Roth-era) after-tax contribution to the plan or a contribution to a "Roth Option" within the plan. See IRS's "Types of Employee Contributions"
here.
The old-style after-tax contributions come out tax free (like Roth contributions), but their earnings are taxable (like traditional contributions). The good news is that a fairly new (2014)
IRS rule makes it easier to roll over those pre-tax contributions (still not the earnings, though) into a Roth IRA. So once you leave the company, you can move the pre-tax money into a Roth making future earnings tax-free.
The Roth Option that's attached to an employer plan (401k, etc.) is still part of that plan and still has the same withdrawal restrictions as the pre-tax (salary reduction) money. It's not the same as a Roth IRA. As the
IRS writes: "
the same restrictions on withdrawals that apply to pre-tax elective contributions also apply to designated Roth contributions."
It is important to keep the vehicles straight: there are employer plans (401k, etc.) and IRAs. Roth is a modifier meaning "after tax, and earnings may be tax-free". There are Roth 401(k)s (and 403(b)s, etc.), and there are Roth IRAs. In employer plans, in addition to Roth option contributions there are "classic" after-tax contributions.
The total contributions you make to an employer plan via pre-tax and Roth Option moneys is limited to $18K (plus possible catch-up). This is independent of whatever you contribute to an IRA, which has its own limit.
The "classic" after-tax contributions are not restricted by this limit. They are subject to a total defined contribution limit (employer plus employee contributions) of
$53,000.
In case I've been as clear as mud, here's M*'s writeup of pre-tax vs. Roth vs. after-tax:
Should You Make Aftertax Contributions to Your 401(k)?