A recent update included rating restricted rooms. You can now create Match and Challenge rooms that restrict player entry to a range based on the overall player rating which is seen in your Player Profile page. Spectator entry is not restricted. Rating changes while in the room playing, don't matter, it is only entry that is tested.
Rating restriction can be used to keep a certain level of competition which makes for more even play.
Though tournament rooms also have the rating restriction shown in the room list, that is just an annotation of the rating restriction specified for the tournament which is only tested when signing up.
April 21, 2015