The power commander unit interconnects between the ECU and injectors.
It allows modification of injector open timing (and therefore amount of fuel) beyond that of the ECU.
This allows tailoring of air/fuel ratio for optimum power.
If the ECU was fully re-programmable, there would be no need for a power commander.
The PCIIIUSB is the 3rd generation of Power Commander that uses a USB interface to upload/download maps for air/fuel ratios.
The earlier PCII, PCIII, and PCIIIr use a serial port for communication.
The PCIIIr has the capability to change ignition timing for each cylinder (r for race).