There are three key parameter in any control loop 1. Process Variable(PV)(often it is called controlled variable or CV) 2. Manipulating Variable(MV) 3. Setpoint(SP).
For an example, for an electric water heater temperature controller, the heated water temperature is PV, the required hot water temperature that is set by user is SP. Based on the difference between PV and SP controller changes(manipulates) the electric flow(Amp) in the heating coil. Here the Amp is MV.
Override control: For a over ride control system, there are more than one controller(normally PID) with different PV. But there MV is same. So multiple controller output will try to change one single MV(control valve opening in most of the cases). Now, which controller output will control the MV that is decided by a Selector(either low selector-selects the lowest signal from all controller output or high selector). That selected signal is passed to the MV for actual action.
Override control is widely used for machine protection for compressor, expanders etc.
Here the bypass valve is getting signal from both I/L pressure controller and O/L pressure controller, but the valve opening is actually controlled by the controller which gives lowest output.