6-cell Kakuro run summing to 21

A 6-cell Kakuro run summing to 21 has only one valid digit set: {1, 2, 3, 4, 5, 6}. The order along the run is still up to the perpendicular runs that intersect each cell, but no other digit can appear anywhere along this run, so all five other digits are immediately ruled out as candidates for every cell.

Locked digits: 1, 2, 3, 4, 5, 6. 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: 7, 8, 9. None of these digits appears in any valid combination, so you can rule them out of the run's cells immediately.