This part is based around the diverter valve section. The diverter valve is directional water device that only appears in combi boilers. It allows the hot water to be directed to the heating system (radiators, etc.) or to the domestic hot water system (taps, etc.). This feature is what seperates a normal boiler from a combi boiler due to the boiler being able to provide heating and hot water without requiring a cylinder.
We recommend that all parts are fitted by a fully qualified gas safe certified plumber.