Form 1099-DIV does not show short-term capital gain, and there is no box between boxes 1b and 2a. That line was inserted by your financial institution for your information. It is not part of the official Form 1099-DIV, and you do not enter it in your tax return.
I assume it's telling you that $139 of the total dividends came from short-term capital gains. But since it's not part of the official Form 1099-DIV, there is no official definition of what it means. If you want to know for sure, ask the financial institution that issued the 1099-DIV to explain it.
