Hub

The Hub is a section of Aperture Science that is featured exclusively during the Cooperative Testing Initiative. Through the Computer Intelligence Training and Enrichment Center Human Test Subject Research Center, or CinTech (pronounced Sin-Tech), Atlas and P-Body can access the various test chambers that are featured throughout the coop campaign.

It features a large screen that displays the total steps players have taken since starting co-op, as well as total portals fired and other statistics. Additionally, it has Aerial Faith Plates, Hard Light Bridges, and Panels that allow for easy traversal of the chamber.

Trivia

 * There are 2 different map files for the hub. mp_coop_lobby_2 is the original version of the hub present in the game at release, while mp_coop_lobby_3 added
 * mp_coop_lobby_2 is only used when playing with a partner who doesn't have the Peer Review DLC installed. Since the PC and Switch versions of Portal 2 bundle Peer Review with the base game rather than being a separate download, this is only possible in the Xbox 360 and PS3 versions of the game. On PC, the map can also be loaded through the developer console, and still functions as expected.
 * There is no mp_coop_lobby; unused files indicate that this was an early version of the hub which was replaced later in the game's development.
 * As the host (Atlas/player 1), to reset your progress so that your testing partner can experience the story in the Hub from the beginning, use the console to enter in mp_mark_all_maps_incomplete. You won't lose your achievements or your accumulated stats (portals placed, footsteps taken, etc.), but after resetting the level with, all of the test chambers will be closed off and you'll hear GLaDOS's lines again. If your friend was further along with some of the sections done, you can use mp_mark_course_complete and then 0, 1, 2, or 3, depending on what their last completed section was, to make your completion progress roughly equal. Finally, use changelevel again to load the map with the changes applied.
 * There are small speakers with the label 409 in the chamber changer for every course (including art therapy), which can play 3 Turret voice lines at random. ("Noooooo!", "Hooray!" and "Goodbye!")