Not sure it is explained correctly in the manul so here is it once again :
- Each target has a certain number of points worth. That number is decided at the death of the target. It's 150 for an egg, 175 for a dretch, 1 for an human, 3 for an human with at least 600$ worth of equipment etc ...
- Each target keeps track of who caused the damage it receives. For example, a dretch knows it has been damaged for 10 points by player A, 10 points by player B and for 20 points by player C ( the dretch healed in between fights )
Now, you just need to share the points worth according to the ammount of damage dealt. Since an human with poor equipment is only worth one point, it's the alien who deals the most damage who gets the point but for a human worth 2 points, it's the 2 biggest damage dealers I guess.