Form 1116 -- Requires Detailed Explanation. Explanation of what?

During the TT final check of my tax forms, TT identified a final Check This Entry review item. The associated TT comment is: Form 1116 (Copy 1) -- Explanation Stm: Detailed Explanation must be completed. Enter your explanation here to complete the required statement. It must show the other deduction items for line 3B column a. TT does not provide further explanation as to what line 3B column a represents in Form 1116. What are they looking for?