The 90 gram CO2 adapter screws onto a paintball gun ASA.  This is the part of the paintball gun that a CO2 bottle or HPA bottle mounts to the gun.  The 90 gram adaptor allows for one time use 90 gram CO2 cartridges to mount to the gun.  Each 90 gram will fire about 100 shots before running empty.  Some paintball gun packages include the 90 gram adapter with the guns.  These adapters can also be removed if you want to use a standard refillable paintball co2 bottle.