How Exactly Do You Define Brewing Capacity?

So, I see a lot of posts around talking about various brewing capacities and giving advice based on said capacity. For example, I hear "anything below 3 bbl isn't commercially viable," and California requires a minimum capacity of 7 bbl.

However, and maybe I'm just thinking about it too much, I can't find any references to how that capacity is defined. I mean, sure, a 3 bbl kettle and a 3 bbl fermenter would logically be a 3 bbl capacity, but what about a 1 bbl kettle and 3 1 bbl fermenters? It seems to me that fermenter space is really the chokepoint, since your beer will be occupying that space for some time. Kettles and mash tuns are occupied for a few hours on one day, per brew.

Capacity almost always refers to the kettle. Widmer, for example, has a 250 barrel system. Running 23/7, it keeps six 750 and six 1000 barrel fermenters going.