That indicates the Year of the deferral for make-up amounts. The employer had to do it that way:

"Beginning with the earliest year, enter the code, the year, and the amount. 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.

A 2022 contribution of $7,000 does not require a year designation; enter it as D 7000.00. Report the code (and year for prior year USERRA contributions) to the left of the vertical line in boxes 12a through 12d."