Level
Often if a pump is taking suction from the bottom of a vessel, increasing level above a medium setpoint starts the pump and as the level falls a lower level stops the pump. In such cases for hazardous processes the common practice would include a separate lower level signal to stop the pump in a manner that is outside the controls provided by an automatic selection.
Of course, if the pump is going into a vessel, then the high level would stop the pump and low level would start the pump.