Lethal vs Non-Lethal Damage
All characters have two pools that measure their vitality — health and stamina.
Health functions as one would expect it to — it is your life force and when a character's health reaches zero, they die. Stamina is a measure of the character's current energy and well-being. If a character's stamina reaches zero, they become too fatigued to carry on and are knocked unconscious.
The two pools are closely related, though in a perfectly healthy character, health and stamina are equal to each other. However, a character's stamina is always less than their current health. This is to emulate the situation that a character who is already wounded would more easily succumb to their wounds after already suffering damage. Meanwhile, health is independent of stamina and damage to a character's stamina has no impact on their health. An example of this is provided below.
Mary is undamaged, and has 50 health and thus 50 stamina. John deals 10 non-lethal damage to Mary. Mary's current health is 50, current stamina is 40. John deals 25 lethal damage to Mary. Mary's current health is 25, current stamina is 25.
Spells with restorative effects can allow a character to regain health and/or stamina. Most spells such as these tend to restore one or the other, though some will restore both. When healing only one pool, it will have no effect on the other pool. So, healing a character's health will leave their stamina unchanged and vice-versa.
Mary's current health is 25, current stamina is 25. Abby casts Heal, restoring 10 health to Mary. Mary's current health is 35, current stamina is 25. Abby casts Energize, restoring 20 stamina to Mary. Marry's current health is 35, current stamina is 35.
Note that Mary's stamina cannot rise above her current health value.
When dealing damage, most spells/attack options at base deal lethal damage. Almost all attacks can be done in a non-lethal manner, converting the normal damage done into non-lethal damage. Some attacks can ONLY deal lethal or non-lethal damage.
Characters that are considered non-living only have a health pool. Meaning any non-lethal damage they take subtracts from their health the same as lethal damage would. Examples include undead, mechanical constructs, elementals, etc. It should be noted that non-living characters tend to have higher resistances to certain types of damage or even immunity to certain types of damage or effects.