Honour system:

An "honor system" is employed whereby the players rely on each others honesty to admit to being hit,

Because unlike paintball, the plastic pellets stop on hitting clothing. Depending on the muzzle velocity of the gun and distance from which a person is shooting, the person on the receiving end of the shot will usually feel the impact but the pellets will sometimes not be felt by a player at very long ranges, hence the importance of Marshals.