Originally Posted by PSP 2012 Rules
3.8.1. A paintball is live from the time it is shot from a paintball gun in the possession of an active player until the paintball comes to rest.
3.8.2. Any live paintball striking and breaking on a player or any equipment in a player’s possession, or any mark indicating such a strike occurred, is a hit.
3.8.3. An invalid hit is any hit that a judge observes is not from a live paintball striking and breaking on a player. Only judges will remove invalid hits, except that a player may remove an invalid hit on the lens of their goggle system with the permission of a judge.
Now that I have a computer here is the actual rule from PSP.