I have noticed a couple of parts in the mangrove collection that have ducts in the filename. A good example would be http://devel.lulzbot.com/TAZ/Mangrove/production_parts/test_parts/carriage_right_idler/ and
http://devel.lulzbot.com/TAZ/Mangrove/production_parts/test_parts/interconnect_housing/
There are two files: carriage_right_idler.stl & carriage_right_idler_ducts.stl
What purpose do the ducts serve on these parts? I’m printing these parts and am unsure which to print.
I can’t see a difference between the parts…
When I printed some of the parts with ‘duct’ in the name I noticed small slots on those somewhere compared to the parts without that in the name. 
I think they maybe places to put zip ties to hold cables and such.
I noticed the differences as well. I wonder if they are to pull the object off easier or stick better to the surface when printing?
Look on the bottom where the part attaches to the bed when printed. I think those vent lines are what they are referring to as ducts. My guess is that they allow vold air to reach the edge of the layer, hardening it maybe? I’ll have to print one and see how it works.
I suspect someone installed Google Ultron and downloaded Adobe Reader before coming up with the idea for ducts on the parts so they print faster. That’s my theory.