W2 Box 12a - 12d are just the box numbers. They don't have to be filled in. But if they have a Code (1 or 2 capital letters) and amount you need to enter it. Don't confuse the lowercase letters a-d with the uppercase Codes A, B, C, D, DD, etc. The lowercase a-d are just part of the box label. You don't enter them. You can enter the Code and Amount in any box 12.
Here are the instructions for W2s. On page 31 there is a chart for what all the codes mean.
http://www.irs.gov/pub/irs-pdf/iw2w3.pdf
