CSR is basically TrueSkill, but how it's adjusted in various playlists is different. In some of the "less competitive" playlists, your TrueSkill is adjusted in a FFA way (like Arena in Reach originally was), so players place 1-8 based on your score and are adjusted accordingly. In the other playlists (like Castle DLC), they are adjusted in a Team way, so either your team wins or doesn't, and your TrueSkill is adjusted based on the current difference between the teams' TrueSkills. The FFA method converges you to your ideal skill much, much faster than the team method. For how much faster, see: http://research.microsoft.com/en-us/projects/trueskill/

I think the Bulletin before the most recent one outlined which playlists are updated FFA and which are updated by Team. Generally playlists with Objectives are Team-based, with the exception being Action Sack.