updated definition

based on #1

docs/schemas/core/IfcProductExtension/Entities/IfcZone.md

@@ -1,6 +1,6 @@
 # IfcZone
 
-A zone is a group of spaces, partial spaces or other zones. Zone structures may not be hierarchical (in contrary to the spatial structure of a project - see _IfcSpatialStructureElement_), i.e. one individual _IfcSpace_ may be associated with zero, one, or several _IfcZone_'s. _IfcSpace_'s are grouped into an _IfcZone_ by using the objectified relationship _IfcRelAssignsToGroup_ as specified at the supertype _IfcGroup_.
+A zone is a group of spaces, partial spaces or other zones. These spaces may or may not be adjacent. A zone does not have its own shape representation. Zone structures may not be hierarchical (in contrary to the spatial structure of a project - see _IfcSpatialStructureElement_), i.e. one individual _IfcSpace_ may be associated with zero, one, or several _IfcZone_'s. _IfcSpace_'s are grouped into an _IfcZone_ by using the objectified relationship _IfcRelAssignsToGroup_ as specified at the supertype _IfcGroup_. For example, a zone might be used to represent an apartment as a group of spaces.
 
 > NOTE  Certain use cases may restrict the freedom of non hierarchical relationships. In some building service use cases the zone denotes a view based delimited volume for the purpose of analysis and calculation. This type of zone cannot overlap with respect to that analysis, but may overlap otherwise.