This article contains spoilers. Read at your own risk!
It might spoil your gaming experience when first playing the related games.
Peer Review is a free DLC made for Portal 2 aimed at expanding upon the co-op aspect of the game.
Adding a challenge mode for both single-player and co-op maps, it allows a player to race certain maps trying to place the least amount of portals in the shortest amount of time as well as compare their scores with other users.