How much adaptation an exercise produces relative to how much recovery it costs, used to decide what belongs in a program and what does not.
A hack squat and a heavy barbell back squat can build quadriceps similarly, but the back squat also taxes the lower back, the trunk and the nervous system. Same stimulus for the target, more fatigue overall, so the hack squat has the better ratio if quadriceps size is the goal.
The ratio is specific to the goal, not fixed. For a powerlifter the back squat has an excellent ratio, because specificity is the point and the fatigue buys the skill.
Use it when a client is short on recovery: cut the exercises with the worst ratio for their goal first. That is usually the heaviest, longest-range, most systemically taxing lift that is not their priority.