If a value can exist in only one row or column within a block, then the value cannot lie anywhere else in that row or column outside of that block.
Consider the sudoku below.
the center block
, the candidate 8 only exists in
r4c5 and r5c5
. Regardless of which cell the value 8 is in, we know there cannot be another 8 that exists in
. So we can remove the candidate 8 from
the cells in column 5 that are outside the center block
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