Rottenstone (red) is 2,000 grit for polishing.
Rottenstone (white) is 6,000 grit. Also called Talc.
Pumicstone (grey or gray) which most people confuse with rottenstone is 900-1,200 grit depending on how it is ground.
Now that is what it is. As to the use; it is for "Polishing" a finish on a frame.
Shellac or Lacquer when shot (sprayed) ends up with a texture of 180 grit, which is very satin to mat. Polymer dispersing agents are put in to cause the finish to "flow" which leaves the finish closer to 400 grit or shiney.
But, it isn't very smooth. The perfect finish is the smoothness of 4-6,000 grit, but with a sheen more akin to waxed 1,200 grit (satin).
The way to get that with gold leaf is to 1) water guild and polish your gesso and boul. Then polish the leaf into the boul. Or 2) guild which ever way you want, then polish out the finish coats of shellac or lacquer.
Along the way, with the carnuba wax and the pumic or rottenstone, the cracks, corner, nooks and crannies get a layer of wax & stone which is similar to the generations of polishing wax and dust that gives the old frames that "Patina". So to simulate age, framemasters use final coats of wax and stone.