re: Salary of $115,627 needed in order to qualify for a mortgage on a typical American home

I define it as a self sustaining geographic area of commerce.
It don't see it as 1 street (i.e. Wall Street). Or 6 blocks.

The "wake" created from the CBD is exactly what you see in Gwinnett County. Rail lines. Airport. Multiple large hospitals.
It is essentially the surrounding area supporting the district that gives the centrality.
Before you know it, you have a school system the size of a small city. You have dozens of offshoots of medical facilities that support each hospital. All of that is created by the CBD.

If 4 Fortune 500 companies in a county isn't enough to differentiate Gwinnett County from Atlanta, then I don't know what is.