In box/line reduction, the candidates of a digit in one row or column all lie in the same box, so the row or column gets that digit inside the box. Every other cell of the box loses the digit as a candidate. The hint in the app calls this technique "Locked candidates (claiming)".
Before the step
Hint in the app
Locked candidates (claiming): in column 3 the 1 can only be inside box 4. So the 1 can be removed from the rest of the box.
After the step
From the Medium level on, a puzzle in Sudoku by Ibexo can need box/line reduction.
Sudoku by Ibexo gives this hint on the puzzle you are playing, in three steps: where to look, which technique applies and why, then the move.
