Motorised valves (MV) or Zone Valves are used to divert the flow from the boiler to either the DHW cylinder, the CH radiator circuits or both. In some cases the CH circuit may be divided into 2 or more zones, each requiring an MV.
Two port MVs are either open or shut. I most cases they open when powered up and close automatically when power is cut off. When open they operate a micro switch which will in turn operate the boiler and pump.
Some MVs have 3 ports (pipe connections) and can divert the water flow to either or both of the outlet ports. This allows one MV to control heating and hot water, instead of two separate (2 port) MVs. Most 3 port MVs have a mid position option which allows flow to both circuits simultaneously, but some (diverter valves) allow flow to only one or the other at any one time.