Someone receiving healing may instantly spend a hit die to recover that many HP or may recover one hack damage. Healing may also typically be used for damage. If the target is capable of taking healing for damage (such as if they're undead), then they must roll their largest hit die and take that much holy damage. This does not consume any of their hit dice. If a creature that's unconscious receives healing, they are awakened.
Damage that is healed more than 24 hours after it was dealt doesn't heal as fully as when it's fresh. All HP can still be restored, but visible scars remain. These scars have no mechanical effect.