True, I implicitly included the MTTR in the "severity", but this is actually a different thing (severity is more about the impact radius).

But I don't think it changes my point: knowing what/how Google Cloud designs regions or zones is still an implementation detail, what matters is what MTTR they are targeting and this should be known ahead of time.

There are so many "implementation details" that customers are not aware of, because they are always changing, non contractual, or just hard to make sense of, what matters is meaningful abstractions.

I am not saying it's OK if the zones are in the same building or not, I don't know and I was really surprised when I discovered this a few years ago. But this information gives you a mental model of "what could go wrong" that is biased towards some specific risks, and in my experience, relying on these very practical aspects make the risk analysis and design decisions harder to make.

Otho, one thing that may be problematic too (and biasing) is that the common understood definition of a "zone" is the one people know from AWS, so using the same term without being very explicit about the differences will also lead to incorrectly calculated risks. I find the public documentation of Google Cloud too vague in general (and often ambiguous).