The attribute gml:id supports provision of a handle for the XML element representing a GML Object. Its use is mandatory for all GML objects. It is of XML type ID, so is constrained to be unique in the XML document within which it occurs.
<complexType name="CompositeSolidType"> <complexContent> <extension base="gml:AbstractSolidType"> <sequence> <element ref="gml:solidMember" maxOccurs="unbounded"/> </sequence> <attributeGroup ref="gml:AggregationAttributeGroup"/> </extension> </complexContent> </complexType>