A GML Object Collection is any GML Object with a property element in its
content model whose content model is derived by extension from gml:AbstractMemberType. In
addition, the complex type describing the content model of the GML Object Collection may
also include a reference to the attribute group gml:AggregationAttributeGroup to provide
additional information about the semantics of the object collection. This information may be
used by applications to group GML objects, and optionally to order and index them. The
allowed values for the aggregationType attribute are defined by gml:AggregationType. See 8.4
of ISO/IEC 11404:1996 for the meaning of the values in the enumeration.