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.
<element name="AbstractMetaData" type="gml:AbstractMetaDataType" abstract="true" substitutionGroup="gml:AbstractObject"> <annotation> <appinfo>deprecated</appinfo> </annotation> </element>