A dun will always have a dorsal stripe, not countershading. That registry is for duns and buckskins, so buckskin is the only one of the acceptable colors that may or may not have a dorsal stripe (though if it were just a buckskin, not a dunskin, it would only be countershading).
Sorry. This is weird for me to try to put into words. It makes sense in my head.
Ok, that is what I was wondering. One of those 'they will always haves' deal. So, if a dun has a dorsal stripe, but no other prim markings, is it automatically a buckskin? If a buckskin has a dorsal stripe, is it considered a dunskin?