Maintenance
From Halopedia, the Halo wiki
Maintenance areas are numerous in UNSC vessels. In case of on-ship attacks, the accessways can be used to flank enemies.
One such example was the Maintenance accessways onboard the Halcyon-Class Cruiser the "Pillar of Autumn, in which the Master Chief along with other Marines used the accessways 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.
