Matchmaking in Rainbow Six: Siege is randomized. There is no such thing such as a playlist. You are paired with a random team together with random strangers in random modes on a random map. The difference between Casual and Ranked sessions are massive tough. Before able to join a ranked match, a player must reach level 20.
Casual's objective locations are pre-determined and random. For ranked, teams could take a vote where they would like the location of either the objective or their spawn point (for the attacking team).
Players are awarded with rank based on team's performance, not Kill / Death ratio.
Teams of equal skills and intelligence might find themselves fighting in a long session. In order to secure a session win, a team must have two rounds lead over the opposing team. Otherwise, the game will just pit the teams in overtime rounds until someone takes the lead on the ninth round.
If you find yourself bombarded with all the scores in your HUD, ranked sessions does not populate your screen with numbers and instant gratification notifications. Reinforcing that wall is your job, son.
Due to the competitive nature, there is no Kill Cam highlight. This would reveal the location of the opposing teams and that will put the other team in an advantage.
Even with the lesser amount of numbers and notifications on screen, the renown points in completing a ranked session is actually doubled compared to casual. Never judge the book by its cover.