From Halopedia, the Halo wiki
Maintenance areas are numerous in UNSC vessels and can be used for easy passage through the ship. Some maintenance corridors have inter-floor ladders. In case of on-ship attacks, the corridors can be used to flank enemies.
John-117 used several Maintenance corridors on board the Halcyon-class light cruiser UNSC Pillar of Autumn to attempt to stop the Covenant from gaining control of the vessel, as well as to make his way to the Lifeboats.
Maintenance access ways are marked with a green triangle on Halcyon-class cruisers' doors. The Maintenance crew are likely in green or orange fatigue uniform as well.
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