Code BB means contributions to the designated Roth account in a 403(b) rather than in a 401(k), but it's the same idea with the same reporting requirements.
Note that if the contributions are to a payroll-deduction Roth IRA, not to a designated Roth account in a 401(k) as you stated, these would not be shown on your W-2. With a payroll-deduction Roth IRA, the employer is simply depositing your own personal Roth IRA contributions on your behalf and it is not really an employer-provided retirement plan.
https://www.irs.gov/retirement-plans/plan-sponsor/payroll-deduction-ira
