From Halopedia, the Halo wiki
Rations are canned or pre-packaged food for military personnel that is accessible and easy to prepare on the battlefield. Utilised by both the UNSC and the Covenant during the Human-Covenant War, soldiers of both cultures generally regarded their ration packs as foul-tasting and awful.
Rations allow military personnel to eat in dire situations, such as being in a crashed Pelican or surviving through enemy patrols. Each UNSC Marine was equipped with a ration pack. An example of a ration pack is Meal, Ready-to-Eat.
Other issued rations are small, gray, cylindrical tins containing set ration meals. There were five of them in the armory on Cairo Station. During the Battle of Mombasa, Marines stored rations in their outposts before abandoning them. During the Battle of Installation 05, rations were found among the stolen UNSC properties in the Jiralhanae armory. Other examples include special forces emergency bars
Rations can also be used in disaster relief situations. For example, during the First Battle of Harvest, ration packs were distributed to the colonists of Harvest after the planet's entire surviving population evacuated to the capital city, Utgard.
The Covenant also used food rations. Acts of cowardice in a battlefield could lead to the rations of an individual being temporarily revoked as a punishment. First Lieutenant Melissa McKay described Covenant rations as a tube of bubbling paste, that "smelled like old socks wrapped in rotting cheese".
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