One thermostats controller contains two thermostats settings
Normally open contact:
It is commonly used to monitor the fan filter, heat exchanger or close the circuit to output signals when the thermostats exceeds the set value
Normally closed contact:
It is often used to connect the heater and disconnect the circuit when the thermostats reaches the set value. The heater and refrigeration equipment can be switched independently to avoid thermostats deviation caused by traditional switching.