What the heck are junk miles?

As a coach my benefit to an athlete is more often telling them what not to do rather than always what they must do. When it comes to running mileage, my opinion is that consistent volume is always the first priority and that a training plan needs to be based around frequency, intensity, time and type. So if higher mileage (within reason) is always good how does one define "junk miles"? Rashelle Brown (2017) on active.com defines junk miles as "the ones you run that don't produce a specific physiological benefit." Contrary to what one might think tempo miles run too frequently are what she considers junk miles and not the slow training runs. Athletes too often have the impression that all the