What is a single stage vs two stage vs multi stage vs emergency heat and aux heat? Please be gentle. :confused:

Single stage means that the unit only has one mode of operation. It is on or off.
Two stage vs multi stage
That means that the unit comes on in different stages. Usually one half the power on first and if the thermostat still is not satisfied the second stage comes on.

Emergency heat and aux heat
This is a twist in terms usually reserved for electric/heat pump use.
Emergency and aux heat can be the same item just with a different name. It is usually the second stage in a heat pump system or the second stage in a gas/oil system.

Aux heat comes on if the regular/first stage heat cannot keep up with demand.

Emergency heat comes on in case of a failure of the first stage or if the first stage cannot keep up with demands.