I've noticed most of the products out there(here too) don't specify the grit scale they use. I assume that Japanese style water stones use JIS but other products can be a mystery. Personally I prefer real measurements to arbitrary scales, in this case everything should be in nominal microns. At least then I only have to experiment with type of grits, bond type(hard/soft, film, metal plate) feel, and longevity. Instead trying to match grit size-progression between different brands of flat stones, powered wheels, films, rods, and strop compound.
I just found this page comparing Japanese, USA, and European grit scales.http://www.uama.org/Abrasives101/101Standards.html
it's a bit technical but the charts are good, as you can see they are all about the same below 300 grit, but in the fine grits they are all over and not linear (kind of like the mohs hardness scale).
There are also different scales for coated(film, belts, sandpaper) vs bonded(stones) and loose grits.
Long story short: a grit number has a max & min micron size(the d94%-d3% charts) and a target range for the average size(the d50% charts). ANSI is an old scale developed around natural grits and thus has a much wider tolerance range for each grit than the newer F&P Euro or JIS grades. If something is labeled in microns only it is generally in a very tight range.
As you can see, a bonded/loose ANSI-600 averages about 9-12 micron
a JIS 1000 about 11-13 micron,
The euro F600(bonded is F, coated is P) is about 8-10.5 micron
Likewise ANSI 1200 at 3-4.5 microns is about JIS 3500