Exhaustion is when a player chooses to continue consuming stamina even though their character has exhausted their native pool and their allowed measure of Fatigue. When characters gain this condition, the character is no longer able participate in combat and is unable to move save for a withdraw action.
If a character is actively involved in combat when they reach exhaustion, they must withdraw immediately or risk collapsing and becoming Helpless. If a character becomes Helpless, they can be killed unless others intervene.
Helpless is as it sounds: the character may be conscious or unconscious, mute or able to speak, incoherent or able-minded but in any case is unable to defend themselves or prevent others from taking action either for or against their person.