From Halopedia, the Halo wiki
Across the surface of Forerunner Installation 00 is a network of access points, teleporters designed to transport Sentinels to a newly deployed Halo installation in order to finalize preparations. In April 2559, the deployment of Installation 09 activated the Ark's access points. The Covenant splinter group, the Banished, used one of these access points to instantaneously send reinforcements to the ring during their battle against the UNSC forces of the Spirit of Fire. Realizing this, Marines led by Spartans Douglas-042 and Alice-130 fought their way to the access point, using a commandeered Scarab to neutralize most of the surrounding Banished forces. Alice and Douglas then proceeded through the teleporter to assist the UNSC forces on the ring and set an explosive charge to destroy the access point behind them, halting the Banished reinforcements.
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