According to Form 1116, Foreign Tax Credit, page 15
https://www.irs.gov/pub/irs-pdf/i1116.pdf
Lines 3a and 3b
Some deductions don't definitely relate
to either your foreign source income or
your U.S. source income. Enter on lines
3a and 3b any deductions (other than
interest expense) that:
• Aren't shown on line 2, and
• Aren't definitely related to your U.S.
source income.
Line 3a.
Enter the following itemized
deductions (from Schedule A (Form
1040)) on line 3a.
• Medical expenses (line 4).
• General sales taxes (line 5).
• Real estate taxes for your home
(line 6).
If you don't itemize deductions, enter
your standard deduction on
line 3a.
Line 3b.
Enter on line 3b any other
deductions that don't definitely relate to
any specific type of income (for
example, the deduction for alimony paid
from Schedule 1 (Form 1040), line 31a)
