Rainbird on 15 May 2011
"Every videogame is exclusive to a subset of all platforms, where each platform in the subset is capable of playing the game.
If a subset of platforms exist such that the game can be played on all those platforms, but also on a platform outside of that subset, then that the game is not exclusive to that subset."
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I think that about covers it for a start at least. 
Although it does beg the question of where emulators fit into all this.
