The game currency is Renown from xingwang's blog

Additionally, Ubisoft has announced that support for the game will continue in 2017, with another four DLC packs to be released throughout the year, set in Spain, Hong Kong, Poland, and South Korea respectively.  

Rainbow Six: Siege does feature microtransactions, but they don't affect the core gameplay. The game currency is Renown, which is earned through normal play and can be used to unlock custom characters and weapon attachments. Alternately, R6 Credits can be bought with cash to Rainbow 6 Credits unlock characters and weapon skins instantly, and to also speed up the leveling process for a limited period of time.  

Playing Rainbow Six: Siege again has been an interesting experience. What I love about it is that it's very different from most run-and-gun FPS games. It features objective-based missions where each team of five takes turns respectively attacking and defending. Since everyone only has one life, you have to play really conservatively. There's no rushing in and gunning down all and sundry – try doing that, and you'll get eliminated in very short order. Instead, you have to advance slowly and carefully, checking all corners and angles for the enemy, and making sure that you cover your teammates. It's a tactical and cerebral experience that's a far cry from the likes of Call of Duty: Infinite Warfare, and Titanfall 2.