Understanding fridge wiring diagram
The defrost heater is clearly marked in the diagram, so it is extremely unlikely the "compartment heater" is used for defrosting. Most probably the compartment heater is for what used to be known as the "butter keeper" compartment, which maintains a slightly warmer temp than the rest of the fridge so that butter is not rock hard when you want to use it without waiting for it to soften after taking it out. The "compensating thermo" would be used to generate the right amount of heat to keep the compartment at the ideal softened butter temp regardless of what the main fridge temp is set to.