What is the u.s. armed forces retirement benefit deduction

There is not a deduction or exemption on a federal tax return for military retirement income.  Your state may have a deduction or exemption and that will be covered when you are completing the state tax return.

@gilsspot 

IF this is for an NC tax return.

IF...if the $$ for the 1099-R from DFAS has not already been excluded as being eligible for the Bailey Settlement.

Then

...the $$ in box 2a of a Military retirement 1099-R are eligible to be deducted from NC income (during the NC Q&A portion of the interview), as long as those $$ distributed meet the requirements listed here :

Military Retirement | NCDOR

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The Box 2a value used since "The deduction is only allowed to the extent the payments are included in the taxpayer's adjusted gross income.", and the value of box 2a is what would initially be included in the taxpayers AGI.