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="ColdFrontType"><annotation><documentation><html:div xmlns="http://www.w3.org/1999/xhtml"><p>Concrete Met Object type for cold fronts.</p></html:div></documentation></annotation><complexContent><extension base="womlcore:AbstractLineObjectType"/></complexContent></complexType>