How far fork and shock already sink under your static weight, before any impact arrives. For downhill roughly 25–30% of travel, set through air pressure.
Sag is the preload that lets your suspension work at all: if it already sits a little into its travel while you stand still, it can extend into dips and keep the tyre on the ground instead of merely reacting to hits. Too little feels harsh, too much feels vague. The classic mistake is riding the factory pressure out of the shop; the second is measuring in shorts rather than full kit with pads, helmet and pack — that weight shifts the number. Stand on the bike in attack position, measure, correct the air pressure, and only then set rebound: drop off a low kerb and slow it until the bike returns without wallowing. What often goes unnoticed: change one thing per test run and write every value down. Pro numbers off the internet don't transfer — your setup hangs on your weight and riding style.