Are you taking about temperature in Centegrade?
This article may be useful: http://www.thathomesite.com/forums/l...5111526.html?9
There are two balance points of interest:
1. One is where the HP just makes up for the losses. Basicically it's when the heatpump runs all the time.
2. There is a second point where the heatpump is totally ineffective.
This article Heat Pumps and Setback Thermostats
Suggests that one temperature is around 30 deg F where it starts to lose it's efficiency.
It also suggests that at 15 deg F it shuts off.
So, during the 15-30 range based on this example, the HP and aux heat should both be on.
Below 15, only the aux heat operates.
EMERGENCY HEAT just makes sure the compressor never turns on. It's used when the compressor has problems.
A lot of this depends on the thermostat and the system.
Some of this finnesse may be done automatically by the system. Certain thermostats, can be wired with an outdoor sensor and the balance points are set in the thermostat.