Most paper 1099-R forms have box 16 empty....but some states require a value in that box for the software.
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If box 2a has a non-zero value in it, then copy box 2a into box 16.
If box 2a is empty and 2b marked "undetermined", then you copy the Federally-taxable amount of box 1 into box 16 (usually, that is just box 1....but not always)