Broad rectangular coticules are the exception rather than the rule, large stones like this are the 'first pick' from the available material which may also yield a few smaller irregular stones around the edges.
The oblique facets of the garnets tend to shave the steel rather than scratching it, leaving a clean, burr-free edge that requires little or no stropping.
Suppled with its own natural slurry stone, this works like a nagura to open the surface and raise a creamy slurry of sharpening grit.
Like wine and people, each piece of natural coticule has it's own individual characteristics, before very long you won't worry about grit numbers and will instead be concerned with technique - the interplay of slurry thickness and pressure to get the desired result from your particular stone.
At the two extremes, slurry with the consistency of double cream and moderate pressure gives the fastest cut, water only and the lightest possible touch gives the finest polish. Using your slurry stone and a spray bottle to manage the consistency and progressing through double, single, full cream, skimmed milk and water with progressively lighter pressure at each stage is the equivalent of working through the grits. The slurry will eventually blunt with use (how quickly this happens will vary from stone to stone) so rinsing the surface periodically and raising a fresh slurry will keep your coticule cutting efficiently.
Note: Slurry stone may vary from image