5-cell Kakuro run summing to 18

3 digit sets sum to 18 across 5 cells. The run on its own is permissive; its constraints become useful when intersected with the perpendicular runs at each of its cells.

Locked digits: 1, 2. These digits each appear in every valid combination, so they each must occupy one cell of the run; useful when a perpendicular run rules any of them out of a candidate cell.

Forbidden digits: 9. None of this digit appears in any valid combination, so you can rule it out of the run's cells immediately.