Last Possible Number
Last Possible Number flips the question around: instead of asking which cell a digit goes in, it asks which digit a cell can take. Pick an empty cell, cross off every digit that already appears in its row, column, and box. If only one digit survives that sweep, that's the answer for the cell.
Focus on one cell
Pick any empty cell. Look at the digits already in its row — those can't go here. Same for its column. Same for its box. Whatever digit from 1 to 9 isn't in any of those three groups can still go here. If only one digit is left, that's the last possible number for the cell.
The only digit
Every cell sits at the intersection of one row, one column, and one box. Count through 1–9: the digits already placed in those three houses can't go here. If eight are already placed, the one missing digit is the answer. No pencilmarks needed — the placed digits alone force the conclusion.
Apply the result
Eight digits are already placed in the cell's row, column, and box, so the ninth is the only one that fits. Write it in — that's the last possible number.