The required n attribute is the number of Id elements in this array.
Source
<xs:element name="FeatureItemIds" type="ArrayReferenceType"><xs:annotation><xs:documentation>The FeatureItemIds element is a list of the QIF ids of feature items that are to be measured.</xs:documentation></xs:annotation></xs:element>
The MeasureSpecifiedFeaturesActionType defines an action that means: measure the FeatureItems whose QIF ids are specified, and populate the corresponding FeatureMeasurements with values.
<xs:complexType name="MeasureSpecifiedFeaturesActionType"><xs:annotation><xs:documentation>The MeasureSpecifiedFeaturesActionType defines an action that means: measure the FeatureItems whose QIF ids are specified, and populate the corresponding FeatureMeasurements with values.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ActionBaseType"><xs:sequence><xs:element name="FeatureItemIds" type="ArrayReferenceType"><xs:annotation><xs:documentation>The FeatureItemIds element is a list of the QIF ids of feature items that are to be measured.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:extension></xs:complexContent></xs:complexType>