Definition
A set performed at the prescribed training weight and effort, as opposed to a warm-up set done to prepare for it.
Only working sets count toward weekly volume. A client who did five sets of squats but three of them were ramp-up sets at 40 and 60 kg did two working sets, and a program that counts all five is overstating the training by more than half.
The line is effort, not weight. Anything at RPE 7 or above with the intended load is a working set; anything below is preparation.
This matters when clients report their week back to you. "Four sets of bench" means one thing if it is 4 x 8 at 80 kg and another if it is 40, 60, 70 and one set at 80.