Short answer: the same as primary vs secondary road. (And tertiary, and service…)
Long answer: fixing part of the issues from Documenting the problems with `highway=path`
For the rest, I completely agree. But many others don’t. My paths get deleted because they are not paths. Not even mappers tend to look at the secondary tags too much.
Many maps will also ignore the secondary tags on paths and show all paths to be the same. Even though there are several categories.
I think an additional problem is that the sac_scale deviated from the SAC scale, adding to the confusion.