The SpecifiedFeatureNominalBaseType is the abstract base type for shape feature nominals for features that are defined indirectly by a specification rather than by a direct shape description.
The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="SpecifiedFeatureNominalBaseType" abstract="true"><xs:annotation><xs:documentation>The SpecifiedFeatureNominalBaseType is the abstract base type for shape feature nominals for features that are defined indirectly by a specification rather than by a direct shape description.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ShapeFeatureNominalBaseType"/></xs:complexContent></xs:complexType>