Which City Objects can be parents?

[GinaStavropoulou]

When City Object types are listed it is not clear which can be parents and which not.
For example in the building-related City objects it is mentioned:

The installations, furnitures, and subdivisions can have as parents a "Building", a "BuildingPart", or a "BuildingRoom".

What about Units and "BuildingConstructiveElement"?

Similarly in the bridge-related city objects:

The installations and furniture can have as parent a "Bridge", a "BridgePart", or a "BridgeRoom".

What about "BridgeConstructiveElement"?

The tunnel-related city objects we have seem more complete :

"TunnelInstallation", "TunnelConstructiveElement", "TunnelHollowSpace", and "TunnelFurniture" can have as parents a "Tunnel" or a "TunnelPart".

Wouldn't it be better to specify which of these are allowed to be parents and which not?

TBD: Can 'Rooms' and 'Storeys' be parents?

[balazsdukai]

The CityGML specs define these (the UML diagrams).
Currently, a few things are under-explained or under-defined in CityJSON, because the reader is expected to be familiar with CityGML.

But long story short, in case Buildings, as Unit can be a parent, a ConstructiveElement cannot (if I understand the UML correctly).
And I think the same applies to the other types as well.
Rooms and Stories can be parents too.