Documentation inconsistencies for IfcSpatialZone

[evandroAlfieri]

Several issue regarding the documentation of IfcSpatialZone, 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 relationship as it is a non-hierarchical assignment in addition to the hierarchical spatial containment within a subtype of IfcSpatialStructureElement.

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 itself can also be referenced by another spatial element using IfcRelReferencedInSpatialStructure.

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 relationship.

Issue 3

Spatial Composition (and Decomposition) concept templates say:

In addition to the identified spatial structure elements IfcSpatialZone 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) and can be aggregated into a functional hierarchy in the same manner as other spatial structure elements with the constraint that an IfcSpatialZone can only be composed under another IfcSpatialZone.

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?