I know that this is an old comment, but I can shed some light on the reason for the subterranean unit hotlinks questions.
Stories are the only place that ArchiCAD can keep 3D elements. We also don't have a "component" function to copy linked groups like SketchUp has. So, in order to have identical rooms that you can edit, they have to be on a story of a PLN or MOD. Many people would refer to an external file, which we've done before, but…
If you reference external hotlinks, and someone changes an attribute, be it material, surface, file, etc., it will NOT change in the other file, and it will show up wrong in the master/host. This type of thing is typically done with hospitality, hospitals, multifamily residential, and senior living projects, which are heavy on interior design, furniture, etc. So we are constantly changing colors, patterns, and other attributes, which causes a huge mess when the drawings don't update correctly (and potential lawsuits).
Synchronizing hotlink attributes has taught us to go underground and keep everything i the same file, even if it looks silly. Beware!
Chuck Kottka
Orcutt Winslow
Phoenix, Arizona, USA
ArchiCAD 25 (since 4.5)
Macbook Pro 15" Touchbar OSX 10.15 Core i7 2.9GHz/16GB RAM/Radeon Pro560 4GB