What's all this talk about preset systems, CO2 tank threads and valves?
A compressed air bottle holds a set volume of air (45 cubic inch, 68, 88 and so on) at a set pressure of 3000, 4500 or 5000 psi. The bottle is screwed onto a regulator (the regulator, on a preset system, is the threaded piece at the end of the bottle that screws into your ASA) that is pre set to regulate the 3000 or 4500 or 5000 psi that the bottle would normally put out, down to anywhere between 450 and 850 psi, depending on the system you choose. In an adjustable system, the user can adjust the regulator's output to whatever he likes, and in many adjustable systems the regulator does not screw into an ASA, but must be mounted either to the gripframe or a dropforward.
I hope that clears some things up for you.