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="AbstractGeometricAggregate" type="gml:AbstractGeometricAggregateType" abstract="true" substitutionGroup="gml:AbstractGeometry"><annotation><documentation>gml:AbstractGeometricAggregate is the abstract head of the substitution group for all geometric aggregates.</documentation></annotation></element>