"The Unknown Citizen" is a parody of the "The Unknown Soldier," a term used to recognize people whose bodies are found after a battle but cannot be identified. The U.S. Army uses metal dog tags to identify soldiers who are killed in action, but these tags can be lost or melted, and sometimes it’s just impossible to locate or identify a person’s remains. In this case, many countries use the concept of the "Unknown Soldier" to acknowledge the sacrifice of soldiers who die anonymously. France placed a Tomb of the Unknown Soldier under the famous Arc de Triomphe in Paris; England has one in Westminster Abbey; and the United States has one in Arlington National Cemetery. Indeed, the epigraph implies that the poem is attached to a fictional "marble monument" dedicated to the UC.
The concept of "The Unknown Citizen" suggests that the lives of many normal people are so conventional and uneventful that they might as well be unknown or anonymous. They’re just an empty suit or a face in the crowd. Of course, it’s only a metaphor. Normal people don’t often die anonymously – though, sadly, it sometimes happens. With his tongue in his cheek, Auden is trying to celebrate or recognize the "sacrifice" of the Average Joe. However, this sacrifice is nothing like a soldier’s, as Auden is well aware. Rather, the Unknown Citizen is praised for being a good consumer, for buying the same things as everyone else, and for not having opinions that might upset anyone. The message to the reader is clear: you don’t want to end up like the Unknown Citizen.