The required n attribute is the number of point set references in the list.
Source
<xs:element name="PointList" type="PointListType" minOccurs="0"><xs:annotation><xs:documentation>The optional PointList element is a list of references to nominal target measurement points for the feature.</xs:documentation></xs:annotation></xs:element>
<xs:complexType name="MeasuredFeatureType"><xs:annotation><xs:documentation>The MeasuredFeatureType defines that a feature is measured.</xs:documentation></xs:annotation><xs:sequence><xs:element name="PointList" type="PointListType" minOccurs="0"><xs:annotation><xs:documentation>The optional PointList element is a list of references to nominal target measurement points for the feature.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:complexType>