Heat Pumps and Electric Heating - What is a multi-stage heat pump? Which thermostat to buy?

Hello -

I have two 3-ton Lennox heat pump units installed on my property. Both approximately 6 years old. One of the units is for the second floor. I also have a Lennon LP gas furnace as backup heat. I am certain that the firtst floor system utilizes the backup heat when temps drop below 32 deg. I can hear the gas furnace kick on. How do I determine if the second floor system also uses the backup heat? There is a switch on the second floor thermostat for Emergency/AUX heat but there ar no ducts (none that I am aware of) leading from the basement to the air handler in the attic.

Also, what does a muti-stage heat pump refer too? Simply that the system has backup heat? Or is it referring to the type of commpressor?

When selecting a replacement thermostat for the first floor system would any thermostat that indicates that it is for a multi-stage w/ AUX heat work? The current thermostat has a C and W1 terminals. A black wire is connected to the C terminal and a white wire is connected to W1. Thanks for any info.

muti-stage heat pump refer too?
This should be a 2 stage compressor. that upstairs unit should have back up electric heat in it . You need it for when the unit goes into defrost.
I dont see how the gas unit can help the up stairs unit.

ED ;)

or it could have 2 compressors with two refrigeration circuits that are staged as needed but that is used primarily in commercial package units.