A quota service is an internal platform service that tracks and enforces storage usage limits across multiple upstream products. When a user on Google's platform has 100GB of total storage, and Google Drive, Google Photos, and Gmail all draw from that same pool, something has to answer the question: "does this user have room for another 2GB video?" That something is the quota service. It does not move bytes or manage files. It owns the single authoritative number that says how much space a user has left, and it must give that answer fast enough that upstream services can block on it without ruining their latency.