I was waiting for DM to offer his insights on it, and my hunch is running the same way. The behavior of the white stone, in shedding grit, mirrors the behavior of a smallish (pocket-sized) 'soft' Arkansas I've got. It's not very aggressive in cutting steel, and it's tendency to dish or gouge easily with edge-leading sharpening leaves me not very anxious to use it. I've fiddled with it occasionally, but have never liked my edges coming off it (burrs like crazy, while not cutting very well, even on simple steels). This may not be reflective of better quality soft Ark stones (it's the only one I've got, currently), so I'm inclined to believe some are probably much better than others. I'm still contemplating buying a bench-size soft Ark one of these days, from a reputable source.
The labelling of the stone as a 'Fine' in your setup may be reflective of it's relative aggressiveness (or lack of same), or maybe just a labelling error in an inexpensive Tri-hone set. I have an old 6" Tri-hone setup that's also 'generic' in it's manufacture with no marked brand on it; I've no idea where it was made, and I'm sure I didn't pay much for it 25+ years ago, when I bought it (I don't even remember where). And in mine, the 'medium' stone in the set (an Arkansas) is not so impressive, as compared to some other medium Ark stones I have. The Coarse stone in my set is very aggressive (probably SiC) and the 'Fine' is a translucent Arkansas that I've been pretty impressed with. I feel like I got lucky with that stone, at least, for whatever little money I paid.
If the 'medium' in your set is a Fine India (or similar AlOx), then the labelling of the soft Ark as a 'Fine' makes sense, in that sequence. It would be the least aggressive of the three.
