An SLA percentage is a downtime budget written as a promise. The number that matters is not the percentage, it is the minutes it buys you, and how those minutes compare to how long it takes you to notice something is wrong. The table below is the whole of it: everything else in an SLA is remedy and measurement language.
Worth knowing
One thing worth checking in any SLA you are handed: what the remedy actually is. Most service credits are a percentage of the monthly fee, capped, and claimable only if you file within a window. A 99.99% commitment with a 10% credit cap is a weaker promise than a 99.9% commitment from a vendor who picks up the phone.