When a character takes damage, they receive a new Injury. Minor injuries, while still serious, heal relatively quickly and do not impair the target much. Severe injuries leave Battle Scars, disable the bearer, and are much more difficult to treat.
Severity of Injury = Outcome on attack roll - Outcome on defense roll (if applicable) + Weapon Damage - Armor. If the attack Outcome - the defensive Outcome is less than or equal to 0, that means the attack failed, and thus no damage is dealt (calculate this before Weapon Damage and Armor are taken into account).