Well, there's definitely no "official" answers in art, but I think of it as just how much you draw. It does mean something on its own even separated from what or how you're drawing, as it gives you ease and relaxation when you draw and an intimate knowledge of what your medium does when applied in a variety of ways. When I advise someone to "get more mileage" I mean fill multiple sketchbooks with who-cares-what just fill 'em!