Читать, что другие пишут, уже не модно?
Although they are clearly designed with each army in mind, this is more from a fluff point of view. It is clear that any army could happily play any mission, indeed this is expressed in the foreword. There is no reason why a Marine army couldn't play the Counter-Attack mission in the role normally filled by Tau, for example.
Миссии для всех, об этом в предисловии к книге написано.