The optional Spatial Domain component of Geometry Hierarchy is mandatory for Quad Tree Related Geometry, and for its component Geometry Hierarchies.
The optional Spatial Domain component of Feature Hierarchy is mandatory for Quad Tree Related Features, and for its component Feature Hierarchies.
The Spatial Domains must be present, so that a producer is forced to define the bounding area that the Quad Tree Related Geometry's (or Quad Tree Related Features') components occupy.