As Rainbow Six Siege operators gain experience, they increase their clearance level. Clearance level will grant you access to more content, like ranked multiplayer at level 20. Also, your clearance level is a status symbol of your skill and experience in the game. Reaching a clearance level of 50 will unlock the “To the Top” trophy / achievement.
At this time, the maximum clearance level is unknown or infinite as players have reported clearance levels of over 200.