From TT's on demand guidance:
Lines 3a and 3b, deductions not definitely related:
Some deductions on your return may not definitely relate either to your foreign income on line 1 or your U.S. source income. Some examples of these deductions might be medical expenses that you deducted on Schedule A, line 4, or alimony that you paid and deducted on Schedule 1 (Form 1040), line 18a. These types of deductions need to be entered on lines 3a or 3b on the Foreign Tax Credit Computation Worksheet.
If you don't itemize your deductions, your standard deduction will automatically be brought to Form 1116.
Describe on the explanation statement what the deduction was for such as the examples given above.
