Maintenance

From Halopedia, the Halo wiki

Maintenance areas are numerous in UNSC vessels. In case of on-ship attacks, the corridors can be used to flank enemies.

John-117 used several Maintenance corridors onboard the Halcyon-class Cruiser UNSC Pillar of Autumn to stop the Covenant gaining control of the vessel, along with evacuating to the Lifeboats.

Maintenance accessways are marched with a green triange on Halcyon-Class vessels, and the Maintenance crew are likely in green uniform aswell.