Uptime measures how often a service is available when you need it. A figure such as 99.9 per cent sounds high, but it still allows a number of hours of downtime across a year. What matters is how that downtime falls, and how quickly problems are fixed.
For a business whose tills, dispatch or customer portal depend on a system, outages cost real money. It is worth asking what uptime a provider commits to, how they monitor it, what happens during maintenance, and how they communicate when something goes wrong. No system is available every moment, so recovery plans matter.