Zone valves are also known as mid position and diverter valves. A zone valve has a simple open and close function to stop the flow of water going to different ports of the central heating system (zones).
Mid position valves can supply to 2 zones at the same time (Y Plan). Common systems using zone valves are called S plan or Y plan.
Most central heating systems use valves to control the flow of water to different zones. They have a ball inside with a motor attached and move to either stop or allow the flow of water through to the different zones for radiators, under floor heating and to hot water cylinders.
Almost all combi boilers have a diverter valve inside that uses central heating water to indirectly heat the hot water. Modern valves are motorized with a lever to manually open in the case of a motor failure. The early zone valves were manual operated only, sometimes known as hot or not.