Box-line reduction

Box-line reduction (sometimes 'claiming' or 'locked candidates type 2') is the mirror of pointing line. If a digit's only candidate cells within a row or column all fall inside a single box, then the digit must come from one of those cells — and can be removed from the rest of that box.

Like pointing line, it's an elimination technique. It tightens the candidate field rather than placing a digit directly.

When to look for it

Scan each row and column for digits with very few candidate positions. When those candidates cluster inside one box, the technique fires.

How to apply it

1. Pick a row or column and a digit.
2. Find every cell in that line where the digit can still go.
3. If all those cells share a box, eliminate the digit from the rest of that box.

Example