Attack points are the only thing that restrains a faction from continuously attacking other factions. It might occur that a faction has far more active members than another faction, but it would be unfair to give that faction the advantage of being able to keep attacking until they eventually win. Attack points will reach zero eventually. Considering most factions start with 3 attack points, this will be sooner than later. Once all factions have used up all their attack points, new points are given to all factions.
Every factions gets attack points equal to the number of planets they own.
An additional rule that may be added by the game creator is a maximum time period every faction has to spend their attack points. When a faction did not spend all their points before the end of the period, they loose the additional points. This prevents lamers from slowing down or even stopping a game. It can also be used to speed up the game a bit and encourage the less active factions to spend more time on the game, or move to another game. When a game of SC has been played, one of the loosing players must report the loss.