Is there a logic behind when the auxillary heat cuts on with a hest pump?

I have heat pump and the auxillary heat seems to come on at sporadic times. I thought that it would just cut on when the heat pump needed help to get up to a certain temperature.

Also, when should I use the emergency heating?

Thanks.

The auxillary heat will come on to suppliment the heat pump whenever the temp in the space is 3 degrees cooler than thermostat setting. If you have a multiple zone system, the aux. heat may come on line after a preset time no matter what temperature the zone has reached. The aux. heat also comes online anytime the heat pump defrost cycle is initiated.
The emergency heat should never be used unless you are having a problem with your heat pump.
These answers assume that your auxillary heat is electric resistance or heat strips. There will be other times when auxillary heat will come online if you have a fossil fuel based auxillary system.