The g5 had a removable caddy with the air compressor and waterpump and electric cabinet. I think the idea was that it would be lighter to transport and also the possibility that you could just borrow a caddy if your machine broke and you be up and running again.
It has been replaced now with g4x next generation as PFT had too many part numbers for different machines and the G5 wasn't a big seller. It couldn't do any more than a g4!
The g4 is all you really need, theres nothing complicated on it and it just does its job, the parts are cheap and the quality and reputation speaks for itself
