Tier 2 · first asked for by Moderate
Box/Line Reduction
If every home for a digit on a line lies inside one box, the digit leaves the rest of that box.
Also called Box/Line Reduction, Claiming, Locked Candidates Type 2.
The mirror image of a pointing pair. Take a digit missing from a row, and find every cell on that row which could still take it. If they all fall inside a single box, then the digit is somewhere in that overlap — so it is nowhere else in the box.
Pointing looks out from a box onto a line; claiming looks along a line into a box. Both are the same underlying observation, that a digit confined to the intersection of two units is excluded from the rest of both.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it rules out · 5 a candidate that goes
What it proves. In column 3, 7 can only sit in R7C3, R8C3, all inside box 7, so 7 leaves the rest of that box.
Builds on Pointing Pair/Triple.