These accessories allow a split-system heat pump to work with a gas or LP furnace. The accessories control the operation of both pieces of equipment to accomplish two things: 1) That the furnace and heat pump don't operate at the same time, and 2) That the heat pump stops operating when it gets too cold outside, and the furnace runs instead.

Alternatively, instead of purchasing these products you could instead purchase a Honeywell Vision Pro 8000 Thermostat with optional outdoor temperature sensor which will accomplish the same thing.