Damage (hit points)
Levelled characters can sustain far more damage than non-levelled. This is due to training, resilience and an indifference to a "hurt." Things that might disable a more common person are merely shrugged off.
Sufficient damage acquired in a single blow may cause a wound or result in an injury. These additional factors exist to represent the multiple effects of damage, where the loss of hit points aren't merely a "wearing down" of the combatants.
When an injury occurs, bones have broken or some form of internal rupturing has occurred. This is a moderate trauma at the worst; in a medieval or post-medieval setting a severe internal trauma would surely lead to infection and death. It's understood that "injuries" are something the character will come back from.
Damage also measures a character's nearness to death. As damage accumulates, the character is driven towards negative hit points, and thereafter to unconsciousness. This last indicates when a character has reached a point of maximum exhaustion, without actually dying.