Hitch

Air bags generally fit between the axle and the frame , between brackets or often right above the springs themselves, and can be inflated to pressures from 5psi to 100psi. The stock spring pack is designed to carry the GAWR of the rear axle but if loaded close to that the truck may be sitting a little low so the airbags could be used to correct that and level the truck and trailer. I think generally people install them so the can exceed the GAWR.