During their E3 conference, Ubisoft announced that the upcoming beta for the latest entry in the Rainbow 6 series will be available for all platforms on September 24th. At this time, there is no info on how long the beta will last. Siege releases shortly after the start of the beta on October 13th, so the beta could possibly run till the release of the game.
Also announced during their conference, all Xbox One owners of Siege will also get access to Rainbow 6: Vegas 1 and 2 for free. This is sure to sweeten the deal for any fans of the series. The two game modes that will be included in the beta are the PvP mode shown off in earlier trailers, as well as Terrorist Hunt. Fans of the series were clamoring for Terrorist Hunt (myself included), so it is a relief to see it being included in this new entry.SOURCE