A band, on a metric, indicates a minimum and maximum safe value. For instance, if you alarmed when your latency is below 50ms or above 150ms, that'd be your band. Being out of band is being out of that 'safe' window.

It's a totally valid and common jargon phrase from the web services world, apologies though, I assumed that it was wider jargon than it turns out it is...

And the band could be the expected value range, as is commonly shown on many gauges by having a section colored.

There's no language rule that a phrase can have only one meaning. You seem to be taking this too precisely, as the earlier explanations of there being two interpretations are quite entirely reasonable.