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="MultiSurfaceType"> <complexContent> <extension base="gml:AbstractGeometricAggregateType"> <sequence> <element ref="gml:surfaceMember" minOccurs="0" maxOccurs="unbounded"/> <element ref="gml:surfaceMembers" minOccurs="0"/> </sequence> </extension> </complexContent> </complexType>