Reference Guide for Common Codes found in Form W-2 Box 12
Ref Pub 4012, pg D-7.

A   Uncollected social security or RRTA tax on tips
B   Uncollected Medicare tax on tips
D   Elective deferrals to a section 401(k) cash or deferred arrangement
E   Elective deferrals under a section 403(b) salary reduction agreement
G   Elective deferrals and employer contributions (including non-elective deferrals) to a section 457(b) deferred compensation plan
J   Nontaxable sick pay
P   Excludable moving expense reimbursements paid directly to employee
Q   Nontaxable combat pay
T   Adoption benefits
W   Employer contributions (including amounts the employee contributes through a cafeteria plan) to the employee's health savings account
AA   Designated Roth contributions under a section 401(k) plan
BB   Designated Roth contributions under a section 403(b) plan
DD   Cost of employer-sponsored health coverage
EE   Designated Roth contributions under a governmental section 457(b) plan
FF   Qualified Small Employer Health Reimbursement Arrangement (QSEHRA)