Limits the maximum pressure in a circuit.
Generally set 15 bar above the working pressure of system.
Back pressure - Changes in pilot drain or backpressure will generally be added to the set pressure; making a sequence valve.
Direct acting - quick response, low flow, leak free.
Pilot operated - high flow, internal or external leakage.
Unloading valve - e.g. relief valve that switches to minimum pressure for start up.