For what's its worth, I can tell you first hand that in USFS Region 2 (includes Colorado) that as soon as you drop a blade (or a tiller or a compactor bar) you are moving from over-the-snow travel to either "building road" or "grooming".
We groom cross-country ski trails for a Nordic Club, and are currently waiting on USFS analysis/decision as to whether we can groom a section of road that is regularly used by snowmobiles, but is not groomed by a snowmobile club. I can travel that road in our snowcat, but using a blade or dropping an implement is whole different ballgame in the eyes of the Forest Service. This particular road is already compacted most of the winter (so from a common sense standpoint what difference does it make?) but it is the use of machine that changes the regulatory situation. If anyone cares I can go into the background, it has to do with an agreement that USFS Region 2 reached with US Fish and Wildlife Service over Canada lynx.