There are more than 20 different tax items that might be listed in box 12, each has its own letter code. Most W-2s seem to allow 4 items in box 12 (sometimes called 12a, 12b, etc.). But "12b" is not the code. The code is one or 2 capital letters. For example, code "D" is contributions to a 401(k) plan, and code "AA" is contributions to a designated Roth account at a 401(k).
HSA contributions have code "W".
You should enter all the items from your W-2, if you have more than one box 12 item, enter them all with their correct letter code.
