A Part can be engine or airframe mounted components that are add on items to the primary component and vital to its operation. For example a carburretor would be a part of the engine. A wheel and landing gear would part of the airframe. A stall warning device would be a part of the airframe and so on. An Accessory would be something like a wheel spat or new design of stall warning device. It is not vital to the safe/legal operation of the aircraft.