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="AbstractMetaDataType" abstract="true" mixed="true"> <annotation> <appinfo>deprecated</appinfo> </annotation> <sequence/> <attribute ref="gml:id"/> </complexType>