Definition
The heaviest set of an exercise in a session, usually a single hard set that the rest of the work is built around.
The common structure is a top set followed by back-off sets: one set of 3 at 90 percent, then three sets of 6 at 80 percent. The top set provides the intensity and the progress marker, the back-offs provide the volume.
It is a clean way to program an intermediate. Only one number has to improve week to week, and the back-offs are calculated from it, so a bad day scales the whole session down automatically instead of forcing the client through weights they do not have.
Keep the top set honest by capping it. "Work up to a set of 3 at RPE 8" stops a competitive client turning every Tuesday into a max attempt.