I've always described the amounts that it refers to. As you've surmised "A" refers to column A, and 3 is for line 3. If you look at form 1116 it seems to require an explanation for entries on line 3b, not 3a. There are circumstances in which you would need to include an explanation for amounts on line 3a, but those circumstances don't include the standard deduction. In this case I would say something like "Line 3b - deductible part of self-employment tax: $XXXXX." If you want to you can add that the self-employment tax is from Schedule 1, Part II, line 15, but I generally don't.