Carrier Performance EDGE Owner's Manual page 52

"Auxiliary heat on" is displayed (heat pump and Hybrid Heat
systems only)
Auxiliary heat on applies only to the heat pump version. If it is on, you
will see auxiliary heat on. When it is displayed, it means the system has
determined that it's too cold outside for your heat pump to heat the
house without help, so it's getting help from the electric heaters. This
happens automatically.
"Em heat" is displayed (heat pump and Hybrid Heat systems only)
Em heat is the display term for emergency heat and applies only to
the heat pump version. When emergency heat has been turned on,
Em heat is displayed. You have to turn on emergency heat manually -
it doesn't happen automatically - so if you see Em heat on the display,
it means somebody has turned it on.
There is a power outage
If you lose electricity, you won't have to reprogram the thermostat
because your settings are retained in its memory.
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