The required n attribute is the number of external file references in this list.
Source
<xs:element name="ExternalFileReferences" type="ExternalFileReferencesType" minOccurs="0"><xs:annotation><xs:documentation>The optional ExternalFileReferences element is a list of references to additional files that specify the measurement method, e.g. ASTM standard.</xs:documentation></xs:annotation></xs:element>
The id attribute is the QIF id of the action method, used for referencing.
Source
<xs:complexType name="ExternalReferenceMeasureFeatureMethodType"><xs:annotation><xs:documentation>The ExternalReferenceMeasureFeatureMethodType references an external metrology method for measuring the feature.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="MeasureFeatureMethodBaseType"><xs:sequence><xs:element name="ExternalFileReferences" type="ExternalFileReferencesType" minOccurs="0"><xs:annotation><xs:documentation>The optional ExternalFileReferences element is a list of references to additional files that specify the measurement method, e.g. ASTM standard.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:extension></xs:complexContent></xs:complexType>