Only contributions that are pre-tax deferrals and (if permitted under the 403(b) plan) Roth 403(b) contributions should be reported in Box 12 of the Form W-2.
Any other contributions – for example, employer contributions or mandatory employee contributions under a governmental plan – are not reported in Box 12. An employer may, but is not required to, report those contributions in Box 14 (Other) of the Form W-2.
