He (as well as I) was referring to the desired threshold of perceptible texture or detail in the shadows, versus blank pure black. But I'll let Doremus explain it himself in detail if he wishes. But some photographers, especially studio ones, prefer to give a margin of safety to their
exposures, and hence place low shadow texture on Zone III. I don't want to confuse you about this so early in the game; but that advice might cause overexposure problems with certain films like TMax, which are better served by Zone II placement. In any event, many studio still life practitioners didn't pay much attention to the Zone System at all because they knew from experience how to keep the contrast in reign using softer lighting. You'll do fine with a bit of practice.