It's a DLC that includes competetive multiplayer skins. Nope, can't use them in Single or Coop. You can access skins by joining any server and choosing option to select skin. Not much to say, it's just skin. Multiplayer community is still alive, so it might be still worth it. I sure like GI Sniper skin. But most people just use Ghille Suit anyway.

The "Allied Reinforcements Outfit" packages is a quite a good-looking skin content and adds a more options for the times you play on the multiplayer side of the game. All three character models have unique traits and the level of details of their clothing or accessories is impressive. I would suggest getting this pack on a sale, due its relatively high price.