To me, close-up is any distance where active focus isn’t needed for a sharp view.
The point of the game is to minimize using your accommodation to see stuff closer than your relaxed eyes sees, while providing lots of opportunity to use active focus.
What do I mean by relaxed eyes? Well, when your eye is totally relaxed (no ciliary muscle use), you see as far as your natural eye sees. For myopics, that is usually close, mine for example have a relaxed eye distance of 32cm or so. Any closer and it’s blurry unless I accommodate. Any further and it’s blurry unless I active focus. Our goal with EM is to push that distance out to nearly infinite naked eye.
So you want your relaxed eye distance to be just short of whatever you’re looking at for the dual goal of active focus opportunity and not using your accommodation.
Glasses shift this focus point via refraction. Negative lenses push the effective relaxed point away from you. Plus glasses pull that point towards you. In my case, -2.5 diopter glasses put my relaxed eye point at 2 meters, and I can active focus out to nearly infinite.
So I define “close up” as any distance where you have to accommodate to see sharp. For me, naked eye, 33cm (1 foot) is not closeup because my eye is fully relaxed. 25cm is closeup (requires 1 diopter of accommodation). If I put on my -1 diopter PC glasses, I see sharp relaxed out to 50cm. With those, the 33cm is now “close up” as it requires a diopter of accommodation. My current normals of -2.5 diopters have close up being anything under 2 meters (6 feet).
Does that make sense?
Close up, at least the meaning relevant to EM, is just any distance requiring accommodation, based on your eyes and whatever glasses you are wearing at the time.