Source: wikipidia
AHC = Active Height Control
The AHC system adjusts the vehicle height automatically with air leveling control and damping forces continuously according to driving conditions.
The AHC system is composed of an automatic air leveling system and a four-wheel independently controlled semi-active suspension system. The air leveling system is equipped with two stand-alone type air springs on rear two rear wheel, an air compressor, a potentiometer height sensor which measures the relative distance between the body and wheel, and on/off solenoid valves. It aims at the maintenance of the design height against the payload variation on vehicle rear side.