What Is A Busy Scene?

What is consiered to be an "active" or "busy" scene for purposes of computing bit rate and storage? A school corridor is a good example that is pretty much the same, no matter where you live. During class breaks or lunch.... chaos. But when school is not in session or classes are going on, almost no activity.

Is there some way to calculate low, medium and high activity in a FOV?

One in which a majority of pixels are changing?

How is that captured or measured in design? Over what period of time?