What is the difference between a Cloudflare queue and a 503 Service Unavailable error?

While both result in unavailability, a Cloudflare queue is a temporary holding pattern for requests during high traffic. A 503 Service Unavailable error indicates that the server is intentionally unavailable (often due to maintenance or overload). The queue is a buffer; a 503 is a direct signal of a server issue or intentional downtime.