From Halopedia, the Halo wiki
A Class Seven service corridor is a type of ship structure present on some UNSC vessels.
According to Cortana, there was at least one of these onboard the Halcyon-class UNSC Pillar of Autumn, which ran along the ship's dorsal structure. After the ship was set to explode on the surface of Installation 04, John-117 utilized this corridor when he was making his escape. This passage led to a section of a parked M12 Force Application Vehicles, and further down, a hangar bay where at least one GA-TL1 Longsword was present.