The Vokera part is based around the diverter valve section. The diverter valve is a device that only appears in combi boilers. It allows the hot water to be directed to the central heating system (radiators, etc.) or to the domestic hot water system (taps, etc.). This feature is what separates a normal boiler from a combi boiler due to the fact that a combi boiler wouldn't require a cylinder to provide heating and hot water.