The Turbo Tax form tells me to enter box 12 from W-2. My W-2 has a box 12a and a box 12b. I don't know which/what to enter?

[SteamTrain]

..................the software labeling of box 12...... as 12a, 12b, 12c,12d...etc is only labeling to distinguish separate lines "IF NEEDED"

So you edit your W-2, go down to the box 12 area, and look at your W-2 to see how many entries it has for box 12.  For every line with a code letter at box 12 on your paper form, there must be a $ amount alongside of it.  And that is what you enter on the "software " form...for as many box 12 lines (a,b,c,d) as needed.

IF your "software" W-2 has a code letter on any box 12 line, that doesn't exist with a $ amount on your paper W-2, then that code letter needs to be deleted from the "software" box 12, and the $$ amount left blank.....i.e., no box 12 code letters should exist without a real  $$ amount next to it.