This is what the IRS instructions for that form say:
Line 31
Enter the smaller of line 20 or line 30.
Note: Generally, line 30 will exceed line 20 only if you have U.S. capital gains or qualified dividends that are subject to the capital gain rate differential (figured in the Worksheet for Line 18).
https://www.irs.gov/pub/irs-pdf/i1116.pdf
