Players 9-11 are hostile to everyone, so you can use them if you need a spare computer to kill something, but generally it's a better idea to try to avoid using them. Players 9-11 are not affected by triggers and therefore can't run AI Scripts or anything. They also don't have their own player slots so you can't use them in Melee games. It's best not to use the players unless you really need to.
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I believe p 9-10 are spells and 11 is critters or something, something... -_-
Critters, resources, Disruption Web, and Dark Swarm are owned by player 12. I don't know if and for what StarCraft does with players 9-11, that would be interesting to look into.