Heat pumps are just air conditioning equipment that can transfer heat from outside the building to inside and vice-versa. There will be a outside condenser with it's circuit for the compressor load and condenser fan. The inside unit is typically an air handler with an evaporator coil for heating and cooling using the freon gas. The inside air handler will also have a section of strip heaters made out of nichrome wire. These heaters are used to heat the supply air for the building when the heat pump cannot supply enough heat (or if selected on the thermostat). The heaters also assist in defrosting the outside condenser coil during the "reverse cycle", when the heat pump runs both the compressor and the strip heat and spins the electric meter like crazy.
The code does require that the equipment supplier provide labels for both the inside and outside unit, so you can properly wire them.
