Look at the example here:
https://www.forbes.com/sites/kellyphillipserb/2014/02/11/understanding-your-tax-forms-the-w-2/#40a60...
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12a, 12b, 12c, and 12d are just ways to indicate extra rows for box 12.
....only if there is a $$ amount alongside a particular box, would you worry about a code...in the example above, that has a code D, DD and P.
But your paper W-2 may have different codes in one or more of those boxes....and if the boxes are empty, then you should not have a code listed/selected at all.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
