The front of the unit carries a selector with three positions for Heating / OFF / Cooling.
OFF
In the OFF position, the input voltage is physically separated from the output voltage.
Heating
If the room temperature falls below the selected setpoint, the heating contact will close (cooling contact open). If the room temperature exceeds the selected setpoint, the heating contact will open and the cooling contact will close but remains inactive because the selector is set to "Heating".
Cooling
Action reversed.