The EPS Interface Unit acts as a liaison between the DTI chip (the EPS system's brain) and the shifters. It receive impulses from the shifters and converts them into digital signals then sends them to the Power Unit for processing. It also receives a power level signal from the Power Unit to let the rider now when the battery needs a recharge. This is done by a small optical LED window on the side of the interface. Think of it this way, if the Power Unit/DTI processor is the EPS brain, then the Interface Unit is the nerve center. No choosing to do, once size fits all.