First it was on UPlay only, but now, Ubisoft has decided to extend the Rainbow Six Siege Starter Edition sale to Steam and Amazon as well. Get you wallet out and be prepared to drop $15.

The Starter Edition gives players full access to the game, including all missions, multiplayer maps and cooperative Terrorist Hunt. The only difference between the starter pack and the full game is that players get two operators at random (either Smoke, Fuze, Mute, Sledge, Ash or Rook), and then 600 R6 credits that can be spent on new operators, weapons or gear. Eventually, everything can be unlocked and purchased with R6 credits or in-game Renown. But be warned that prices will be a bit steeper than if you had purchased the full game (12,500 vs. 500-2,000 Renown for operators). 

Once the deal is over - Ubisoft again is not saying when that will be - players will still retain access to the game and everything they have earned or purchased.

Oh look, an FAQ if you have specific things you want to know.