Symbolism, Imagery, Allegory
The Library is a huge, unwieldy model with tons and tons of information in it. It also contains a smattering of human inhabitants, who run around the Library doing silly things like killing each other. In other words, the Library is a lot like the universe. Borges is even nice enough to come right out and tell us so in the very first line: "The universe (which others call the Library)..." You know the rest. The symbolic significance of the Library isn't exactly hidden here, but hey – this story is tricky enough as it is. It's sort of nice to have a gimme.