Beginners Guide
3000 or 4500psi = bottle pressure
45 or 88cu = bottle capacity
It basically means that the larger the bottle and greater pressure it can take, the more shots you'll get from it (properly regulated of course).
E.g. I use a 68cu (1.1litre) 4500psi system and get about 800-900 shots from it (due to the fact I can rarely fill the bottle to 4500psi, more like 3500psi with my fill rig).
The standard today is 4500psi but bottle size is a "size preference v number of shots" choice.
HTH.