Space Marines Codex is generic though it can create a variety of different types of armies. The others will share similar units but often have different options. The minis are generally flexible, for example the tactical marines are pretty much the same across all the armies.
I'm a bit confused. There is a Space Marine Codex. There are also Ultra Marine, Dark Angels, Grey Knights and others Codexes (Codeci?).
But those chapters are also Space Marines right? So is the Space Marine Codex a 'generic' one and the others chapter specific? Can you use stuff from the Space Marine Codex in a Dark Angels army? Or Dark Angels in a Space Marine Army? (Not as allies, as actual members of the Army?)
(Same question applies to 'Space Marine' minis, can you use them in a 'named' army?)