A single number for how much work a session or a week contained, most simply session RPE multiplied by session length in minutes.
Session RPE is one rating for the whole session on a 1 to 10 scale, taken about 30 minutes after it finishes. A 70-minute session rated 7 gives a load of 490. Add the sessions for a week and you have a weekly load that can be compared with the week before.
The comparison is the point. A useful guideline borrowed from team sport is to keep the current week within roughly 1.5 times the average of the previous four; larger jumps are where injury risk climbs. A client who has averaged 1,800 a week should not be handed 3,500.
It is coarse. It cannot tell heavy squats from long cardio, and the numbers only mean something within one client. Used that way, it catches the sudden spikes that a set count misses.