What are allowed to be root level construction resources?

Originally https://forums.buildingsmart.org/t/what-are-allowed-to-be-root-level-construction-resources/3550

The documentation states, for construction resources:

> A root-level resource (specifically [IfcCrewResource](http://localhost:8001/schema/ifcconstructionmgmtdomain/lexical/ifccrewresource.htm) or [IfcSubContractResource](http://localhost:8001/schema/ifcconstructionmgmtdomain/lexical/ifcsubcontractresource.htm)) is declared within the project by [IfcRelDeclares](http://localhost:8001/schema/ifckernel/lexical/ifcreldeclares.htm) where *RelatingContext* refers to the single [IfcProject](http://localhost:8001/schema/ifckernel/lexical/ifcproject.htm) and *RelatedObjects* refers to one or more IfcConstructionResource, and other root-level objects within the project.

This creates some ambiguities.

1. Is the intention that a root level resource must only be of the class `IfcCrewResource` or `IfcSubContractResource`? Does this mean that `IfcConstructionEquipmentResource` (or any other `IfcResource` subtype) is not allowed to be a root level resource? That is, an `IfcConstructionEquipmentResource` may only exist as a nested sub-resource?
2. If ambiguity 1 is true, that is, only those two classes may be root level resources, does this mean that they are _only_ root level resources, or can they exist again as a nested sub-resource? E.g. IfcCrewResource -> IfcConstructionEquipmentResource -> IfcSubContractResource