A ration card, known informally as a food card, was a 21st century form of identification issued by social workers to residents of San Francisco Sanctuary Districts on Earth. They allowed the inhabitants access to subsidized food and water at any of the distribution points in the district.
They were uniquely assigned to specific residents, but despite this, ghosts, like B.C., stole the ration cards of Sanctuary District A's weaker residents. Following the death of Gabriel Bell, Julian Bashir was able to identify the deceased from his food card. (DS9: "Past Tense, Part I")