Hidden Unique Rectangle
Use solution uniqueness and a strong link involving a rectangle's corner to eliminate a candidate that would create another solution.
All candidates have been entered in the empty cells.
Assume that the puzzle has a unique solution. Candidate 6 has only two possible places in A. If candidate 5 is true in B, candidate 6 is true at the other place in A. Therefore, the two digits can swap into another solution. Candidates 5 marked × can be removed.