A service level agreement fixes the availability and response commitments: how much downtime is acceptable, how fast an incident is acknowledged, and how fast each severity level must be resolved.
Without written targets, support expectations are a source of conflict: the client assumes an hour, the vendor assumes next business day. Uptime figures also drive architecture cost, so the target must be chosen deliberately.
We agree severity levels and response times that match the real cost of downtime for your business, publish monitoring that makes availability verifiable, and prefer an honest target we can hold to an impressive one we cannot.