I think most of it is explained on Page 32 unde the Topic "Resolving Retreast" .
But basically it goes like this;
After all skrimishes are resolved between two players front-line units there are three possible outcomes.
1. Only the Attacker has units left = those units occupy the area where the battle occured.
2. Only the Defender has units left = those units remain on the area where the battle occured.
3. If units survived on both sides, the attacker failed = all of the attackers units have to retreat to one empty or friendly area och the current planet or one planet adjacent to the present one that the attacker has a transport connecting the two planets.
Average values are usually used when you dont have cards on your hand that fit your units in combat or if you just dont want to waste all your really good cards.
It could also be used if you dont have any Combat Cards on your hand in a fight and must retreat to drawing a random card from your combat deck Draw pile.