Claiming
Claiming occurs when every remaining candidate for a digit in a row or column lies in one box. The line must place the digit in that box, so the digit can be removed from the rest of the box outside the source line.
What is Claiming?
Claiming — sometimes called line-box reduction — is the mirror image of Pointing. In one row or column, all candidates for a digit lie inside a single 3×3 box. That digit must occupy the intersection, so the same candidate can be removed from the rest of the box outside the source line.
How to spot it
Scan the rows and columns one at a time and look for a digit whose candidates all cluster inside one box. With pencil marks on, this jumps out — the marks for that digit bunch into a single 3×3 area instead of stretching along the line. Once you see it, sweep the rest of that box and erase the digit wherever it appears outside the source row or column. Claiming shows up a lot in the first half of a solve, when the grid is still crowded with candidates and one clean elimination can open things up.
The logic
Every row and every column needs each digit 1–9 exactly once. If a digit's only candidates in a row are all inside one box, one of those cells has to be the digit — there's nowhere else on the row it can land. But a box only holds each digit once, so once the row uses up that digit inside the box, no other cell in the same box can have it. That's why every candidate inside the box, off the source row, can be crossed off. The same reasoning with a column swapped in gives the column version.