I am not sure what that means in regards to ductwork, and I think that maybe you don't have to worry about this too much. Typically, plenum rated means that the item doesn't have a flame spread greater than 25 and a smoke developed rating greater than 50. Most things in a system are metal and most insulation meets these criteria. I think all it would take is a cursory check on the materials to make sure you are OK.
Electrical cable needs to be plenum rated to be installed in a return plenum. This rating means the same as above.
