If it drops below 67%, the soldier will be Tired after the mission ends -you can still deploy them in this state, but it's a very bad idea, as they'll react badly to more or less literally anything bad happening, and once the mission completes they'll probably earn themselves some manner of psychological problem that will impair their performance in future deployments even once they've rested. Specifically, when you deploy units, their Will 'rots' over the course of a mission.
(In part because a lot of elements of XCOM 2 and War of the Chosen are clearly influenced by Long War, and this system is broadly comparable to Long War's Fatigue system) A new mechanic in War of the Chosen that the manual briefly alludes to, but which is not properly explained anywhere in it or the game itself, is what I call a Fatigue system.
