Step 7 · Medium
What is Box/Line Reduction (a line decides the box)? How to Solve Killer Sudoku
When all of a digit's candidates in a row or column lie inside one box, you can remove that digit from the other cells of that box.
What it is
The reverse of Pointing. If, in a row (or column), a digit can only go in cells inside one box, then that box's copy of the digit must be on that row.
So the box's other cells (off that row) can't have that digit.
Example: in row 3, 8 only fits in cells inside box 2 → remove 8 from the rest of box 2.
How to spot it
Go through rows and columns one at a time and look for a digit whose candidates all sit inside one box.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
In column 1, 2 can only go in R7C1, R8C1, and R9C1. They are all inside box 7. So box 7's 2 must be on column 1, and the other cells of box 7 (R7C2, R7C3, R8C2, R8C3, and R9C3) can't be 2.