Question: what are box 12 codes on w2?
Answer: A — Uncollected Social Security or RRTA tax on tips. ... 
B — Uncollected Medicare tax on tips. ... 
C — Taxable costs of group-term life insurance over $50,000 (included in W-2 boxes 1,3 (up to Social Security wages base), and box 5.
D — Elective deferral under a 401(k) cash or arrangement plan.