The gap between resting and maximum heart rate, used to set training zones that account for how fit a client already is.
The Karvonen method: target heart rate equals resting heart rate plus a percentage of the reserve. For a client with a resting rate of 60 and a max of 190, the reserve is 130, and 65 percent of it gives 60 + 85, which is about 145 bpm.
It is better than taking a flat percentage of max, because a fit client with a low resting rate and an unfit one with a high resting rate should not be given the same number. Using a plain 65 percent of max for both would put the unfit client into a harder effort than intended.
Measure resting heart rate properly: on waking, lying down, averaged over three mornings. Estimate max as 208 minus 0.7 times age, which is more accurate than 220 minus age, and treat both as approximations.