In on-off mode, a controller’s output(s) are turned on / off as the process variable crosses the setpoint, in a manner similar to a central-heating thermostat. There is usually some adjustment of the on and off switching hysteresis either side of the setpoint, to allow for a differential or dead-band. This helps to avoid rapid switching of the output on faster processes.
Some oscillation of the process variable is inevitable when using this type of control, and a larger hysteresis will increase the amplitude of these oscillations.
In dual control applications, it is usually possible to setup a combination of control methods, where the primary control output uses PID and the secondary output uses on-off control.
On-off control cannot be implemented with linear (analog) outputs.