Roth IRA - Designated Roth Accounts

Per the IRS Retirement Plans FAQs on Designated Roth Accounts:

What is a designated Roth Account?

A designated Roth account is a feature in new or existing 401(k), 403(b) or governmental 457(b) plans. If a plan includes a designated Roth feature, employees can designate some or all of their elective deferrals as designated Roth contributions (which are included in gross income), rather than traditional, pre-tax elective contributions.