Here are the instructions for box 11 on a W2.

Box 11—Nonqualified plans. The purpose of box 11 is
for the SSA to determine if any part of the amount
reported in box 1 or boxes 3 and/or 5 was earned in a
prior year. The SSA uses this information to verify that
they have properly applied the social security earnings
test and paid the correct amount of benefits.
Report distributions to an employee from a nonqualified
plan or nongovernmental section 457(b) plan in box 11.
TIP
Also report these distributions in box 1. Make only one
entry in this box. Distributions from governmental section
457(b) plans must be reported on Form 1099-R, not in
box 1 of Form W-2.