I've consulted with many companies trying to save money by moving to AWS because they have twelve 9s of availability. They don't do any redundant deployments because they want to save money.
I keep telling them that the SLA just means that AWS will give them a refund for anything you couldn't use while they have an outage. There's no guarantee.
So they just treat their uptime target sorta like the office and bankruptcy? "I declare 12 9s!"
Like, bruh, you actually have to pay for that if you want it, wtf?
I guess it is a bit like car batteries. A 4 year warranty doesn't guarantee it will last 4 years, it means you get a refund if it doesn't. (Though unless they're idiots they build the batteries so statistically it is somewhat more common to last that long vs dying early).
But that's why you put SLAs into contract language with penalties. Like, say my company pays your company for a SaaS product. And you go down longer than the SLA, we get a discount or something. If it's not in a legal agreement it is meaningless for sure.
Service Level Agreement, typically a legal contractual requirement. SLAs can vary, but a common one is for a service to be online and accessible. Web hosting providers that host web sites for companies typically guarantee that their service has an SLA of 99.5% uptime, meaning any time the service is down, the engineers and incident teams need to restore operational services quickly. A breach of SLA invokes financial penalty to the provider for violating the contract uptime guarantee.
With most pre-written SLAs, the penalty is something like "we'll refund the service cost for the month" at best. So it's "we have a financial reason not to fuck up" not "you will be made whole if we fuck up and your business is down".
The SLAs are also often tied to SLOs (the quality they promise to deliver, e.g. "we promise to be up 99.5% of the time") that are very generous for the service provider. If your critical service was down 3.6 hours in a month, that would still meet a 99.5% SLO. So if your business was down for 2-3 hours per month, that would be a-ok. Only if it was down for say an entire day, you'd get (depending on the contract) typically either a day or a month of service refunded.
I'd take a provider with no SLO but a good track record over someone that offers an SLA. If they fuck up the month of refund is going to be the least of my problems, and if they fuck up repeatedly, I'll have to emergency-migrate away to a different provider either way.