“We guarantee 99.9% uptime” is the number people ask for, and usually the least useful of the three. It describes the system; it says nothing about what happens when your staff cannot work.
Three separate promises
- Response time — how long before a human who can help acknowledges the ticket. This is the one your staff feel.
- Resolution time — realistic only per category. A password reset and a failed RAID controller do not belong under the same number.
- Availability — meaningful only with the measurement window and the exclusions written next to it.
Ask for the exclusions in writing. An availability figure that excludes planned maintenance, third-party outages and “force majeure” without defining any of them is a marketing number, not a commitment.