It's still meaningful to discuss 99.99% on top of things that are around 99.99%. For example, let's say you have a service on AWS and all your clients are on A

It's still meaningful to discuss 99.99% on top of things that are around 99.99%. For example, let's say you have a service on AWS and all your clients are on AWS. If AWS is down, you are down but so are your clients. But your clients want you to be up 99.99% of the time that AWS is up. As long as both sides are aware of the implications, this is fine. As long as you're within the same order of magnitude, it can make sense. If a customer wanted me to be up 99.99% of the time on top of a service that is only up 99.9% of the time, I would push back.