The number of terminals on the thermostat that can be used to individually control separate heating functions. Many furnaces that are called two stage heaters will require at least a two stage thermostat to operate appropriately. You may also choose a multiple stage thermostat when pairing different heating equipment. Such as a heat pump and a furnace.

The capacity, measured in kilowatts, of the heater coil. On average every 1kw of heater coil translates to 3,412 BTUs of heat.

This is the amperage requirement for the electrical circuits. This will determine the size wire, the number of circuits to run, and the electrical panel breaker size required.