Never underestimate the stupidity of some publishers - they added more DRM to the Steam game than the normal one, crazy isn't it?
Well, it will act like a normal version of Clear Sky until you install it five times and then, bam, you've got an error saying you have run out of activations. You can contact TAGES and request more, the information on how to do this is on the Steam forums, and they'll generally give those out - but still, I would have not recommended that version of the game (the boxed version, the one I have, has TAGES but it is only a DVD-check and not the limited installations/activations).