I think one of the best things about Tom's courses is that they are strategic, as designed, with multiple choices (right, left, short, long, lay-up, go for broke, etc.).
So many modern courses present options that qualify their challenges as "strategic." However, many of these strategies have only two results:
1. Lay up, do not challenge the hazard. You are going to be left with a more difficult shot to execute.
2. Hit over the trouble, open the angle, leaving a straightforward approach.
This is Strategic Design 101.
What sets Tom's courses apart, and I think this speaks to the "subtle gray-shaded" area he is talking about, is that his strategic design principles do not always present (or solely present) basic Strategic Design 101. The basic tenets are there, but they have been built upon.
Perhaps call the corollary "Strategic Design 201":
1. Lay-up short or away from the trouble. The shot is on the surface a more difficult one (and likely will be 80-90% of the time), but the player may be able to be on a certain half of the fairway or in a small, quirky spot where there is a clean look to the green. This spot may be tough to get to, or maybe a lucky result, but it gives the shorter or weaker player hope, and increases the fun quotient for those who elect to play safe.
2. Challenge or carry the hazard. There is a clean look to the green from the resulting position, but the green is undulating, or crowned, or small. Certain pin locations might be hidden behind a mound or slope making only a crisp and controlled wedge or short iron the shot of choice.
So, the strategies are no longer only two-pronged, they now branch out like a family tree. As a low-handicap player, I find Tom and Renaissance's designs very difficult compared to the average course because I can never let my guard down.
I thought about the above points in terms of a short par 4, such as #6 or #16 at Pacific Dunes, but I think it applies to all lengths of holes.
This best describes Tom's "gray-shaded area," for me, that each shot and strategy should not be a foregone conclusion, which may be (so far) Tom's greatest contribution to the art form.