I don't think I understand what you mean? I thought it was clear. As a 1 you can imagine an apple within your had at high detail, noting the location of the stem and maybe being able to zoom in on its texture, seeing the striation of its color and perhaps a moisture. Maybe you can rotate it and note the dimple at its underside of slice it and see it's slightly yellow roughly textured crisp inside. As any level between 1 and 5 you lose detail; maybe the shape is imperfect or fuzzy, maybe you can't see the color as clearly or perhaps it's a solid color, maybe it's less of an apple shape and more spherical, the stem may be less noticable or not there at all, you may be able to imagine how it might feel under your teeth but not manipulate the visualization into slices and see it's core or it's seeds. Am I misinterpreting?
Edit: I misinterpreted your comment it seems, I thought you were critiquing the concept itself instead of asking for clarification. Apologies. I do think that the 1-5 scale is entirely arbitrary and doesn't really express accurately the varying degrees at which people are capable of visualizing; like most things, it's probably a spectrum.