The required n attribute is the number of Id elements in this array.
Source
<xs:element name="ReferenceFeatureNominalIds" type="ArrayReferenceFullType" minOccurs="0"><xs:annotation><xs:documentation>The ReferenceFeatureNominalIds element is a list of the QIF ids of associated nominal features.</xs:documentation></xs:annotation></xs:element>
The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="OtherNonShapeFeatureNominalType"><xs:annotation><xs:documentation>The OtherNonShapeFeatureNominalType defines the nominal values of an other non-shape feature.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="NonShapeFeatureNominalBaseType"><xs:sequence><xs:element name="ReferenceFeatureNominalIds" type="ArrayReferenceFullType" minOccurs="0"><xs:annotation><xs:documentation>The ReferenceFeatureNominalIds element is a list of the QIF ids of associated nominal features.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:extension></xs:complexContent></xs:complexType>