Per the IRS Retirement Plans FAQs on Designated Roth Accounts
What is a Roth 401(k) or Roth 403(b)? Is it a new type of plan?
No, it is not a new type of plan. Designated Roth contributions are a type of contribution that new or existing 401(k), 403(b) or governmental 457(b) plans can accept. If a plan adopts this feature, employees can designate some or all of their elective contributions (also referred to as elective deferrals) as designated Roth contributions (which are included in gross income), rather than traditional, pre-tax elective contributions