800 psi output = This is the regulated max pressure going into your Marker.
Almost every bottle regulator has both low pressure and high pressure burst disks.
HP side = Pressure that is stored inside the bottle
A bottle that can be filled to a max of 3000psi will have a 5k HP burst disk installed.
A bottle that can be filled to a max of 4500/5000psi will have a 7.5k burst disk installed.
LP side = This is the pressure which enters your marker when passing through the reg. This will have a 1.8k burst disk to protect your marker in the event of a reg failure.