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In the items you noted, it's not important whether it's Box 12a, 12b, etc.  They are all Box 12 and you want to look for the capital letter code in order to report these items in your W-2.  If you had HSA contributions they'll be in Box 12 with a code W.  That's how you'll enter it in the W-2 section to get the HSA section to show the amount.