Just a number I picked from domain knowledge. Almost all newer heat pumps can still efficiently pull heat out of that temperature, although some more expensive ones can go all the way down to -20 - -30F ambient air temp.
EDIT: If you're in a colder climate, in-ground coolant loops are preferred over an air transfer heat pump.
EDIT: If you're in a colder climate, in-ground coolant loops are preferred over an air transfer heat pump.