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="LineStringInterpolationCurveType"><annotation><documentation>This type is deprecated. WOML line objects no longer have a pre-calculated interpolated geometry as a property, so this type is no longer needed</documentation></annotation><complexContent><extension base="womlcore:LineStringCurveType"><attribute name="samplingFrequencyValue" type="double" use="required"/><attribute name="samplingFrequencyUOM" type="anyURI" use="required"/></extension></complexContent></complexType>