Guages
All regs have a high pressure guage which indictes bottle pressure - i.e. How full the bottle is, 0 to 3000 or 4500 psi.
Adjustable regs have a low pressure guage which shows the output pressure - i.e. the pressure going into the gun, e.g. 0 to 400, 0 to 800 or even 1200 depending on the reg.