It works with the priority setting in locos. A train can have a priority of 1,2(default) or 3. When under AI control the train will choose a path with a marker of the same level if it has a choice of paths. At least in theory that is how it works. It along with track direction markers are legacy items from the early days when AI was called Autodrive. You didn't program the path of a AI train to use, autodrive just tried to make its way as best it could making choices at each junction. Those two markers could be used to tweak the choices the autodrive controlled loco made. I assume they are still around for old routes that have them so the route doesn't show a missing dependency. Autodrive was replaced by the Drive command. I'm not sure that it obeys those markers.
For example, If you wanted a passenger train to stay on the mainline at sidings you would make the loco have a priority of 1 and put a marker of priority 1 on the mainline at the siding. Others trains of priority 2 or 3 would take the siding and allow the passenger train to go by.
William