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="AbstractSolid" type="gml:AbstractSolidType" abstract="true" substitutionGroup="gml:AbstractGeometricPrimitive"><annotation><documentation>The AbstractSolid element is the abstract head of the substituition group for all (continuous) solid elements.</documentation></annotation></element>