Terms and payment
SLA and uptime
A provider's availability promise, e.g. 99.9%, and compensation if it is missed.
An SLA (Service Level Agreement) is an agreement on service quality. Most often it states uptime — the share of time the server works. 99.9% allows up to 43 minutes of downtime a month, 99.99% about 4 minutes.
What matters more than the number is what happens if it is missed: an extended service, a partial refund. If no compensation is written down, it is more a promise than a guarantee.