The development team has scheduled Rainbow Six Siege’s first scheduled maintenance down time on Wednesday, December 16th at 1PM EST. The down time should last approximately an hour, during which time online play will not be available. After the maintenance has completed, exit the game client if it is not already and download the client patch via the Uplay interface.
This is significant because this maintenance period is so that they can apply the first round of updates to the game. This update is supposed to address several game play issues as well as some overall improvements to the game.
You can get the full list of things that have been updated at the Official Rainbow Six forums.
