I really do not know which type i had. They placed plastic on all the celing vents before starting. They cleaned the coils in the air/furnace unit also in the attic.
WELL, there was so much soot that it seeped out from the plastic, covering the vents. Dirt and soot was on the celilings and on the floors, and on bedspreads and carpeting, etc. etc. etc. What a friggin mess. I have not stopped coughing. I have changed my return air duc filter 3 times since these guys finished. (one month now)
They also put a mild cleanser on the coils to keep mildew and mold from getting on them.
If I were ever to have ducts cleaned again, I would plan on being out of the house for days and do more investigating on how this is properly done.