A character’s Stamina Point (SP) maximum is equal to their Endurance (Characteristic) bonus, though it may be modified in other ways. When a character is below zero SP they have a level of fatigue for each point of Stamina below zero. A character may still spend or lose SP even if they have none remaining, but each time they do so they gain a level of fatigue. This can cause them to eventually exhaust themself and fall Unconscious.
Regaining SP
A character regains an amount of SP equal to their Endurance bonus after a Long Rests and regains only 1 SP after a Short Rests (or removes a level of fatigue).