The number of things that can be independently controlled within a process is limited by the process' degrees of freedom. This means that if it becomes necessary to control an additional variable it is necessary first to release a degree of freedom by dropping the control of something else. For example consider the control of a boiler system. Normally the steam discharge rate will be manipulated by a pressure controller to maintain the desired supply pressure. However under certain conditions the supply of condensate returned from the plant may be insufficient to maintain the desired steam pressure and at the same time keep the heating coil covered. In this situation keeping the coils covered and preventing them burning out takes priority over pressure control, and a level controller would be allowed to override the pressure controller and take over the use of the steam discharge rate.
Override control is usually accomplished through the comparison of controller outputs in 'less than' or 'greater than' calculation blocks.