The HPOP builds high-pressure oil which is used to actuate the injectors. The PCM monitors various parameters (engine speed, load, etc.) and adjusts the HPOP pressure via the standpipe and injector control pressure to regulate the amount of fuel injected. It does this indirectly; the PCM doesn't directly control the injectors, but controls the oil pressure that *drives* the injectors. The injectors themselves are electronically controlled but rely on the HPOP for their actuation pressure.
