Designed by GLaDOS sometime during the single-player campaign, they are to complete the Cooperative Testing Initiative in the Aperture Science Enrichment Center, previously uncompleted by human Test Subjects. Gameplay-wise, ATLAS is played by the host of the co-op session (the player who sends an invitation), while P-Body is played by the guest (the player who accepts an invitation). PlayStation 3 players are automatically assigned as the host during cross-platform games, to prevent use of the console by the PC player.