Power Units are a combination of the start solenoid, motor, drain valve, manifold and oil reservoir that drives the hydraulic liftgate system. Gravity down models use the power unit to raise the lift gate platform for loading, then control the gravity down movement of the platform to ground level. Power down models use the power unit to raise the platform for loading, then use it to lower the platform to ground level.  Generally power down units are associated with heavier loads. Units are differentiated based on load size requiring different hydraulic strength, necessitating different motors, controls and reservoir sizes.   Space considerations are also important. Liftgate reservoirs hold the hydraulic fluid that drives the cylinders for raising and lowering the platform.   Two major power unit suppliers dictate the majority of the reservoir shapes and sizes available.   The SPX/Fenner/Stone replacement reservoirs bolt directly to the manifold of their power units, being either metal or plastic poly.   The Monarch/Bucher replacement reservoirs attach with a pressure ring to their manifold and also come in either metal or plastic poly.