Different concepts. This has nothing to do with "in case the power fails."

This has apparently changed since last I went shopping for a heat pump. Used to be, heat pumps couldn't be expected to sufficiently heat a house on the coldest days of the year, so a second additional heat source would have to be installed, which also doubled as a backup in case the heat pump mechanically failed. The setting on the thermostat to engage this second heat source was labeled "Emergency Heat" or "EmHeat", like so:

A better term would probably be Auxiliary or Supplemental heat but the industry uses the term Emergency Heat.

Apparently there are heat pump systems on the market now which can pass inspection on their own with no supplemental "emergency" heater. I suppose that goes hand in hand with the technology becoming more viable in the northern latitudes. They didn't used to be suitable for colder climates but now they are, I guess that means they're suitable for flying solo here. I wouldn't personally do that because A. I like to have the backup in case of system failure, and B. without heat strips or a furnace, the defrost cycle is just...running the air conditioner. The source of defrost heat is your mammalian ass.