I'm not sure that there is. Everything starts out structural, though, so the only detail brushes that exists are the ones that you turn into detail brushes. If you aren't sure either way, just make it the one that you want it to be- nothing can be hurt by something being turned into a detail twice.
"The chief difference between men and women is that men forget, but never forgive, while women forgive, but never forget."
-Thom Merrilin, The Wheel of Time series