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="CubicSplineCurveType"><annotation><documentation>A Simplified version of gml:CurveType with only CubicSpline segments.</documentation></annotation><complexContent><extension base="gml:AbstractCurveType"><sequence><element ref="womlcore:cubicSplineSegments"><annotation><documentation>This element encapsulates the segments of the curve.</documentation></annotation></element></sequence></extension></complexContent></complexType>