IfcSpatialZone and IfcExternalSpatialElement confusion about low to high levels in spatial tree

The documentation states: Spatial Composition
The order of spatial structure elements being included in the concept for builing projects are from high to low level: IfcProject, IfcSite, IfcBuilding, IfcBuildingStorey, and IfcSpace with IfcSite, IfcBuildingStorey and IfcSpace being optional levels. Therefore an spatial structure element can only be part of an element at the same or higher level.

In addition a more general hierarchical tree of spatial elements can be created by using IfcSpatialZone, from high to low: IfcProject, IfcSite, and IfcSpatialZone with IfcSite being an optional level.

Two questions:

  1. Can someone clarify the intention for IfcSpatialZone? Does this mean that IfcSpatialZone must either belong to an IfcSite or an IfcProject and nothing else? What does the second paragraph mean?
  2. Why is IfcExternalSpatialElement not mentioned at all? Where does it fit in the hierarchy? Is it hierarchical? I assume it is, as I haven’t read otherwise. Is it an optional level? I also assume it is. Common sense tells me it is an optional level that must either belong to an IfcSite or IfcProject, but I cannot find anything about this in the spec.