Where 90% is used
Effectively nobody publishes 90% as a customer-facing commitment. Where you do meet it is in internal batch systems, lab environments, and legacy hardware where downtime is scheduled rather than feared.
What it demands
Nothing. At three days a month you can detect an outage by someone walking over to your desk. If a vendor offers you 90%, they are telling you they have no availability engineering at all.
One step either way
Moving up to 95% cuts it to 1d 12h.
Measuring it
You cannot report on a service level you are not measuring at a finer resolution than the budget itself. At 90% the monthly allowance is 3d, so a check interval anywhere near that figure will miss breaches entirely. Work out your own numbers, or monitor it continuously from four regions, free to start.