As far as I'm aware there are no restrictions of the type of food you bring to any of the major MILSIM events. The event organizers are less concerned with what you bring than that you bring enough calories to remain functional the entire event. For more "traditional" events like those hosted by Lion Claws, Black Sheep and Third Coast Airsoft, breaks are built into the event so players have time to rest and eat off the field. For longer 24-40 hour events like MSW, players need to pack their food onto the field and many bring MREs, Mountain House meals and other freeze-dried victuals that they "cook" up using water heated with Jet-Boils or other camp stoves.