At least one "reference" property is mandatory, localizedName is optional. The parameters should primarily be identified by references to extenal catalogs.
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.
Source
<complexType name="GeophysicalParameterType"><annotation><documentation>At least one "reference" property is mandatory, localizedName is optional. The parameters should primarily be identified by references to extenal catalogs.</documentation></annotation><complexContent><extension base="gml:AbstractGMLType"><sequence><element name="reference" type="womlcore:catalogReferenceType" minOccurs="1" maxOccurs="unbounded"/><element name="localizedName" type="womlcore:longLocalizedStringType" minOccurs="0" maxOccurs="unbounded"/><element name="abbreviation" type="womlcore:shortStringType" minOccurs="0" maxOccurs="1"/><element name="aggregation" type="womlcore:TimeAggregationPropertyType" minOccurs="0" maxOccurs="1"/></sequence></extension></complexContent></complexType>