In The Logic of Sudoku, Andrew Stuart introduces a UR method in which corners on one side are known values, i.e. clues, and remaining candidates are prevented from forming a deadly rectangle with these clues. This is his Avoidable Rectangle.
Andrew shares his puzzle maker’s experience with this UR formation, and shows several examples. The simplest case is shown in this slink marked version of Andrew’s first example.
Let’s say that 8 was just added at r3c7, confining 6 candidates to r12, proving N6, implying the naked pair 89 in N. If you are the least bit aware of the avoidable UR, you can now fill the N box.
Additional UR applications apply to avoidable rectangles. Logic provides two examples similar to these:
In Avoidable 1, the eliminations do not depend on which 7 will be in the solution. Additional 7 -candidates in c23 would not be eliminated as demonstrated in Avoidable 2
Here is the box marking checkpoint, before clean up.
Below is the box marking trace.
Tom tells his apprentices that #3 contains a unique rectangle, but he does not hint what kind it is. And he doesn’t tell, in Sudoku Master Class, how to handle such a UR. That’s what happens to apprentices, isn’t it? They are assigned ambiguous tasks without adequate training, and then get all the blame when they mess up.
Even though you may never have seen such a UR, you have some experience and won’t mess up. I will tell you that the target does not emerge until line marking is almost completed. Come back next week with a removal, or at least, knowing where the UR is located.