In 6th edition 40k rule book an army can include allies, or not, according to this table.
First the obligitory. **** you Matt Ward!
Much of it obvious based on what you'd expect but there's few surprises. Doesn't make a lot of difference to playing this game but opens up some story lines or some background. I could have easily put it other rpg forums but there seemed to be more talk of 6th ed here in general.