From Halopedia, the Halo wiki
A mess hall is a room that provides food and drinks for military personnel.
A standard Halcyon-class light cruiser's mess hall is located near the bridge and armories. Rather than having on-board cooks, the mess halls on the Halcyon-class cruiser Pillar of Autumn were equipped with automated food and drink dispensers instead; it is unknown if this is the case on most or all UNSC fleet ships.
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