W-2 Box 12

[sterburns]

Box 12 has the code D then number 21 and the amount.  What does the number 21 stand for?

[JohnB5677]

D21 indicates your make-up contributions for 2021 under the Uniformed Services Employment and Reemployment Rights Act (USERRA).

 For example, elective deferrals of $2,250 for 2020 and $1,250 for 2021 under USERRA under a section 401(k) plan are reported in box 12 as follows. 

D 20 2250.00, D 21 1250.00. 

[SteamTrain]

[Edited]

OK...detailed fully proper response from @dmertz  below:

He understands all the gory-specific-details that the rest of us do not

[dmertz]

"Occasionally, an employer has to make a late makeup 401k contribution for a prior year, so an entry labelled D20, on a 2021 W-2 form, would indicate that such a makeup contribution was made for the year before (i.e. 2020). "

That not how this works.  Code D reports employee elective deferrals.  If the employer failed to include the correct code-D amount on the 2020 W-2, the employer must correct the 2020 W-2.

The D21 indicates a USERRA make-up contribution.  Since it was made in the same year as the year of the W-2, the "21" is redundant and can be omitted.  If there is a separate code-D amount with no year designation, the two amounts can be combined.  (If the USERRA amount was instead for a prior year, the USERRA amount should be omitted.)

https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-userra-and-sscra