From Halopedia, the Halo wiki
The Milwaukee lacked Cryo-chambers due to its advanced slipspace drive. Instead, stadium styled seating was put in a large passenger cabin. The roof of the cabin had a polarizing roof, that could turn opaque or transparent.
By September 2558, the Milwaukee was re-purposed as a large passenger ferry to and from Onyx. Molly Patel and several hundred passengers took the Milwaukee to reach Onyx at this time. During the trip, they received a briefing and presentation about Onyx from Director Hugo Barton.