Documentation inconsistencies for IfcSpatialZone

Several issue regarding the documentation of [IfcSpatialZone](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcSpatialZone.htm), raised by implementers trying to support it.

#### Issue 1
_IfcSpatialZone_ documentation says:

> Physical elements that are referenced by this spatial zone are related using the [IfcRelReferencedInSpatialStructure](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcRelReferencedInSpatialStructure.htm) relationship as it is a non-hierarchical assignment in addition to the hierarchical spatial containment within a subtype of [IfcSpatialStructureElement](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcSpatialStructureElement.htm).

However, _IfcSpatialZone_ inherits from _IfcSpatialElement_ the ability to contain physical elements (i.e., using _IfcRelContainedInSpatialStructure_

**These two scenarios are in contrast. Shall the quoted constrain be enforced?**

#### Issue 2
_IfcSpatialZone_ documentation says:

> The [IfcSpatialZone](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcSpatialZone.htm) itself can also be referenced by another spatial element using [IfcRelReferencedInSpatialStructure](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcRelReferencedInSpatialStructure.htm).

However, _IfcSpatialZone_ inherits from _IfcObjectDefinition_ the ability to aggregates/be aggregated other (Spatial) Elements (i.e., using _IfcRelAggregates_)

**These two scenarios are in contrast. Shall the quoted constrain be enforced?**

Note that the same issue applies to _IfcSpaces_, as _IfcSpatialZone_ documentations says:

> Also spaces, that are referenced by this spatial zone are related using the [IfcRelReferencedInSpatialStructure](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcRelReferencedInSpatialStructure.htm) relationship.

#### Issue 3
[Spatial Composition](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/concepts/Object_Composition/Aggregation/Spatial_Composition/content.html) (and Decomposition) concept templates say:

> In addition to the identified spatial structure elements [IfcSpatialZone](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcSpatialZone.htm) can be used to provide cross domain or functional zones within a project, these elements are included into the hierarchy using the Spatial Containment concept ([IfcRelContainedInSpatialStructure](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcRelContainedInSpatialStructure.htm)) and can be aggregated into a functional hierarchy in the same manner as other spatial structure elements with the constraint that an [IfcSpatialZone](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcSpatialZone.htm) can only be composed under another [IfcSpatialZone](https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcSpatialZone.htm).

A few uncertainties in the above statement:
1. It is not clear what's the subject of *these elements*. If it's _IfcSpatialZone_, then the templates are in conflict with the _IfcSpatialZone_ description, which mandates (?) the use of _IfcRelReferencedInSpatialStructure_ to connect _IfcSpatialZone_ to other elements of the Spatial Structure
2. The text suggests that an _IfcSpatialZone_ can be aggregated with another _IfcSpatialZone_, to create a hierarchy, similar to what's done with Spatial Structure Elements (using _IfcRelAggregates_). This is either i) in conflict with the _IfcSpatialZone_ description, which mandates (?) the use of _IfcRelReferencedInSpatialStructure_ to link _IfcSpatialZone_ with (any) another spatial element, which de facto includes another _IfcSpatialZone_; or ii) missing from _IfcSpatialZone_ documentation, as it doesn't say anything about Spatial Zones being aggregated with each other nor which leaf of the "Spatial Zone tree" shall be linked to the Spatial Structure and how.

#### open questions
The above inconstancies are preventing implementers to have **clear, unambiguous answers** to the following questions:
1. Can _IfcSpatialZone_ be aggregated into the spatial breakdown?
2. Can the same _IfcSpatialZone_ also be referenced by multiple, other elements of the spatial breakdown?
3. Can _IfcSpatialZone_ contain physical elements?
4. In both positive/negative answers of the above questions, how are these restrictions enforced?