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
The Online version only starts out with one Box 12. There is an "+Add a Row" selection underneath that adds a another line...then another...then another...etc. So like if you have something filled in box 12b, 12c, 12d, etc. You can have more than 4 rows for box 12.
