What does box 12a- S mean on w2

[lethielen]

I think it is my 401-K contribution but how do I know?

[DanielV01]

It is a retirement plan contribution, but it technically is not a 401-K (that would be a symbol D).  This contribution is from a SIMPLE or a SEP plan.  For more information on box 12 codes, please see this FAQ:  https://ttlc.intuit.com/replies/3300234

[dmertz]

Code S is for an elective deferral to a SIMPLE IRA only, not a SEP-IRA.

Enter the code S amount only on TurboTax's W-2 form.