How does it work?
The Rainbow Six Siege skill rating system is based on the Trueskill algorithm. It's an evolution of the Elo/Glicko rating systems that works better for multiplayer games (Elo and Glicko are designed for 1v1 games like chess).
Your skill rank is initially computed after 5 placement games, from your skill rating value (for math geeks, the mu variable). Following that, it's updated after each ranked game you play.
Your skill rank is specific to the region you're playing in (Europe, North America, Asia, etc.). Players from different regions almost never play against each other, and since skill isn't an absolute measure but a relative one (it doesn't tell how good you are -- only whether or not you're better than another player), ranks can't be directly compared across regions (RTS and MOBA players call that the Korean bronze effect).
This means that if you move halfway across the world, you will have to go through your placement games again and find out how good you are compared to other players in your new region.
What makes my rank change? What parameters are taken into account?
Exactly two things:
• The relative skill levels of the players/teams in the game.
• Whether or not your team won the game.
KDA doesn't matter. Score doesn't matter. Hashtag MLG 420 noscope montage-worthy plays don't matter. Rainbow Six is a team-based game and as such, the only thing that accurately determines whether you're a good R6S player is your ability to make your team win games.
In addition, if there's a large skill difference between the two teams and the outcome of the game is what the system expects, the skill adjustment after the game will be minimal (if a bronze team loses to a gold team, it's unsurprising and the skill levels won't change much -- now if the bronze team wins, that's a whole different story).
There is, however, one exception to all that:
If you abandon a ranked game (whether it's by ragequitting, AFKing or getting votekicked out), it's always counted as a loss for you, even if your team wins.
Furthermore, if the game server crashes (it still happens from time to time, and we're working to minimize that too), we consider that the game didn't happen. No skill adjustments are made.
For more information regarding ranked play please follow this link.