Riot Games introduces Act Ranks a new rank system as revealed on an official blog post.
Act Ranks are determined by your best 9th win to track your “proven skill,” matchmaking ranks is different than Act Ranks and will continue to reflect your current rating.
Playing games fills up your Act Rank with smaller triangles representing your ranked match wins that can be replaced by your higher rank wins to display your best competitive games. The border also changes as you when you hit certain thresholds: 9, 25, 50, 75, and 100.
Starting in Act II, your friends will be able to inspect your Match History, including both your matchmaking rank and overall Act Rank progress.
At the end of every Act there will be a soft reset in rank and put players in abridged placements; needing only 3 games instead of 5 to display a matchmaking rank with reference of the players’ previous Act MMR. Ranks will also be a conservative placement where “we are confident you are able to compete at that skill level—or higher.”
The start of Act II will likely be on August 4 judging by how many days remain in Act I.