I haven't heard of this. I'm curious as to what this means as well. My first thought would be that clean is meaning it's all good (in-stock parts or no issues).
It means the vehicle is still not scheduled (obviously) but there is nothing preventing it from being scheduled such as constraint items. So it is just a matter of time until it actually is scheduled. Although possible, it doesn't mean you'll be picked in the next round of selections.
Basically, it's telling the dealer there's nothing more he or she can do at this point to speed up the selection process, it's up to Ford at this point.