Damage is what happens to you or your partner when you go too far. It's when, at the moment, you essentially consent to something permanent. Someone may become proud of the damage, but have obvious regrets, as with being branded by a less-than-worthy top or having a crushing-out cigarette destroying a tit.
Some would argue that minor damage involves anything from minor scars up to (but excluding) castration. Major damage is loss of a body part. There is some major literature about this, for example Tennessee Williams and Joe Horton.
It's wanting to hurt/be hurt while hard, while yet restraining yourself out of self-preservation.
Mutual scenes tend to be the most extreme, where permission is granted.
At a certain point, permission is granted to damage by many bottoms.
Note: In many countries, you may be charged with a criminal offence if you cause bodily harm to someone; consent may not be an adequate defence.