From Halopedia, the Halo wiki
Commons R-01, also known as Commons Red, is a generic United Nations Space Command term for one of the commons modules on the port side of an Orbital Defense Platform. It consists of a standard-sized rectangular room, with elevated "islands" regularly spaced to the edges of the room. Security R-01 overlooks Commons R-01.
Cairo Station Commons Red
Cairo Station Commons Red was a port side commons of Cairo Station closest to the bridge. SPARTAN-117 rode past Commons R-01 on one of the station's trams right before the Battle of Earth begun on October 20, 2552. During the battle, Covenant boarders invaded Commons Red, killing many Marines and pinning the survivors down. SPARTAN-117 arrived and helped the surviving Marines repulse the invaders.
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