Purpose of Evaporator Defrost Control Kit

[moonman]

I live in PA and my contractor recently installed a Trane XV95 furnace and a Trane XL15i heat pump with matching cased coil. In the proposal he included an Evaporator Defrost Control Kit (AY28X084) which says "allows low ambiant cooling to 30 degrees". Can someone explain what this kit is used for in a heat pump and how can I tell if the kit was installed.

[temco]

Evap defrost control is a temp sensor mounted on the suction line of the indoor evaporator coil. It shuts the compressor of if evaporator temperature gets to low to protect it from icing up or returning liquid to the compressor during the cooling cycle. This would be installed if you planned on running the unit in cooling when outdoor temp is below 55deg. Usually only on commercial application where there is a indoor heat load during cold weather. Not sure why it would be installed on a residence.
Possibly whoever wrote the proposal got it confused with the heating cycle defrost control that prevents outdoor unit from freezing during heating cycle.