Bike ans Sail,
It's not the federal government but it's the state that you live in that is requiring that box 16 be filled. The states are double checking that they are getting their fair share.....lol Only if withholding for state taxes is in box 14 and a state is in 15 that they are requiring now that box 16 is filled. Use the number from box 2a, if no number is there use the number from box 1.
I am not a professional but after lots of research this seems to be the best response. This is what I did when I filled and I just received my state refund.
