Availability of network access to existing instances.
What you're talking about with provisioning capacity is a totally different matter. Provisioning availability is not guaranteed (unless you purchase reserved instances) and there are frequently periods where certain instance types are not available in certain AZs, though they do try to resolve that as fast as practicality allows them to. It really stinks sometimes though - especially if you get into a situation where something fails in your autoscaling group and there is no capacity available for a replacement instance. Usually you can get around that though by making sure your ASG is set up for multiple AZs, or worst case changing instance types (though that can be problematic in it's own way).
source: I used to work for AWS Support.
