The Crystal Spheres distance is a system to describe a relationship of different Crystal Spheres and describe the difficult of creating magic that uses two or more of them. Closely related spheres are considered to have a shorter distance, described in spherical units (su) where unrelated magic is further away.
The idea of spherical units comes from astronomical units (au) which is the distance between the sun and Earth. It is a very self-centered view of humanity and one that is based on an inconsistent distance (since we get closer and further from the sun based on our orbit) and ultimately is not a good measurement. In the same way, spherical units are one attempt to try understanding the world that will eventually be replaced with something more accurate as the understanding grows and politics are overcome.
Ultimately, I think the types of magic will be eventually organized into a taxonomy like our kingdom, genus, and species. At the same time there will be an attempt to create a chart of all the known spheres. Both of them will eventually fail until we get into a system of related characteristics that can be sliced and viewed in different ways. That will eventually be “these are inert” or “these have a low electrical impedance”.