If you examine the properties of a duct you'll see a parameter called "Reference Level". Do the same for a Duct Fitting and you get a parameter called "Level".
If you choose to create a schedule of ducts or duct fittings a fairly obvious way to sort them is by the level or floor they are related to. Oddly enough there is no access to either Reference Level or Level. Some other elements have "Associated Level" offered as a valid schedule field, not ducts or duct fittings...not duct systems either.
Revit has been plagued with seemingly arbitrary restrictions on what can appear in a schedule from the beginning. For ducts I can see where a riser that spans several floors might not have a very meaningful "reference level" value but it does have one, so I think it ought to show up in a schedule. In fact I frequently realize that a duct references the wrong level because I placed it in a 3D view, particularly risers. If that showed up in a schedule I'd have another chance to catch it and adjust it accordingly.
In my over-simplifying opinion, if a Revit element has a parameter value in the properties palette it ought to be able to show up in a schedule too.