Possibly confusion arises here from darunter meaning below vs. among/amongst. In the following sentences I try to explain how this ambiguity is usually resolved:
1) In den oberen Geschossen wohnten wohlhabende Bürger, darunter Aristokraten.
2) In den oberen Geschossen wohnten wohlhabende Bürger. Darunter Aristokraten.
3) In den oberen Geschossen wohnten wohlhabende Bürger, darunter dann Aristokraten.
4) In den oberen Geschossen wohnten wohlhabende Bürger. Darunter wohnten Aristokraten.
1) resolves darunter as among, though while it may be spoken like this, it may not be entirely grammatically sound. This is also somewhat ambiguous.
2) introduces a distinction between the first sentence and darunter, so darunter is less likely to refer to the previous sentence and readers are likely to read this as below. As with 1) I think 2) is not entirely grammatically sound.
3) This is an example how then is introduced to resolve ambiguous meaning of darunter. This sentence resolves darunter to below and is not ambiguous.
4) With respect to grammar, this is probably the correct one. It resolves the ambuguity by introducing a verb. In this example darunter is unambiguous and resolves to below. Introduction of a verb to signify context does have an exception though - sind. Sample sentence: Sechzig Demonstranden sind vor Ort, darunter sind zwanzig Studenten.
The meaning of darunter is - at least in spoken speech - sometimes a bit ambiguous, but usually clearly resolved by context.
As for reference to an inanimate object I agree with Janka's answer. The previously described (by naming its occupants) stage in the hierarchy is what darunter refers to and could be expanded to darunter in der Hierarchie in this example.