In contrast to the FunctionalEntityType Part 81 does not explicitly indicate that Sub-Assets are possible (compare Figure 24 – FunctionalEntityType overview (<SubFunctionalEntity>) to Figure 21 – FxAssetType overview.

However the References that are used to connect Assets (HasPhysicalComponent, HasAttachedComponent, HasContainedComponent, HasBuiltInAsset and HasPart) are all HierarchicalReferences as HasSubFunctionalEntity is.

In Motion we have use cases, e.g. modulare Drive systems, where we would get a 3-Level hierarchy of Assets (e.g. 1. Modular Drive - 2. Axis Module - 3. SafetyModuleAddon).

We would like to create example Asset topologies in the Motion Annex. Before we do this it needs to be discussed what is the right way to model these type of Devices using Assets (flat list vs. hierarchy).

Part 81 should be more descriptive if hierarchical Assets are allowed and what requirements apply for hierarchical Assets compared to Top-Level Assets.