A Non-Released Platform is where a game is available to be played completely, but it not available as a completely independent title, often bundled with a compilation of other games.
One example is "Portal", which was released for Microsoft Windows as an independent title. Microsoft Windows would therefore be a Released Platform, as would GNU/Linux. However, on both PlayStation 3, and Xbox 360, it is only available as part of "The Orange Box". This makes both the Xbox 360 and the PlayStation 3 Non-Released Platforms for Portal.