A lot of airlines have done refugee flights at one point or another, so I guess they needed a way to tell the ground crew to load whatever special foods were needed.
If they're famine refugees, something like Plumpy'nut[1] maybe. If they're religious refugees, something catering to their religious restrictions. MREs if the refugee operation is being assisted by a military or disaster aid organization. And so forth.
MREs contain a heating packet filled with magnesium pellets and a couple other things, which are used to heat the food when water is added. I'd imagine they don't want those in the cabin, given the gas that is emitted during the heating process.
RFML seems from United Airlines, and seems to mean: easy to digest. No egg, no dairy, probably no meat? Its for people from places where food isn't processed or where they had not eaten much for a longer time, so a change in diet content and quantity may be problematic for the passenger.
I wonder what that is.