even if you knew which system they were referring to, you´d have problems comparing these to stones of other systems.
There are numerous discussions about whether a Norton 8.000 (officially rated 3µm) is finer, or coarser than a DMT 8.000 (3µm, too) and so on.

It takes time and practice to know where in a system a stone or a paper belongs.

Generally the microns help an awful lot. The size of the micron makes things a little more comparable, but just a little