Rainbow Six Siege will soon be getting Alpha packs added to the game, which are essentially loot boxes, and Ubisoft has released an FAQ stating what they will contain, how they can be earned, and the different tiers of content within. The first thing to note is that Alpha Packs will be earned after winning a Casual or Ranked match. However it isn’t as simple as that as Chance comes into play. Chance is awarded after each match with it increasing after matches. If you win a match your Chance rating is taken into account and you roll for a Pack. If the roll falls outside of your Chance rating you don’t get a pack, with additional Chance being added to your account instead. Once a pack is won Chance resets to zero. You cal so purchase a pack for 5000 Renown. Eventually R6 credits will also be usable to purchase.
To open an Alpha Pack you will be required to be online as all of the work is done server side, with all packs containing just one item. These items are broken down into five tiers which are Common, Uncommon, Rare, Epic, and Legendary. If you get a duplicate item then you will receive the Renown value for it instead. While a number of items will be available post launch operators will never awarded in an Alpha Pack.
Source: Official Rainbow Six site.