I'm far from an expert on such things and nor am I in any way interested in knives as a collector or anything like that but my girlfriend got me a Fallkniven F1 and when I'd done about 6 deer with it I decided that it needed some sharpening. I knew Fallkniven did a little sharpening "stone" and as I was impressed with the F1 decided to order one. In the process I also ordered a smaller version of the stone which has just the diamond side and which glues neatly onto the F1 sheath.
Anyhow, all I can say is that the double sided DC4 "stone" does, in my view, a great job of getting my knives as sharp as I will ever need them. It is small and handy and has a diamond side for a knife that is very blunt and then a ceramic side to hone the edge. I gave the F1 a few swipes on the ceramic side of the DC4 and then nearly removed the end of my thumb with it - note to self "using a good sharpening system does not improve your IQ."
This may not impress the knife experts but for someone like me who views the knife as a tool for the job then I would recommend the DC4 as even an idiot who would nearly cut his own thumb off can use one to sharpen a knife, you can't get higher recommendation than that.
The smaller D3T "stone" is just the diamond side of the DC4 but is backed by some glue which allows you to stick it onto the zytel sheath for the F1. It fits neatly and although I don't think the diamond side will give just as good an edge as the much finer ceramic material it would be ideal for touching up the knife in the field. The only slight downside that I can see is that it would be necessary to remove the sheath from your belt in order to use the attached stone to sharpen the knife. However, this wouldn't be such a great hardship and it is good to know that if you've got the knife with you then you have a means of sharpening it along as well.