The optional areaUnit attribute defines the unit name for the SummaryStatisticsAreaType.
Source
<xs:complexType name="SummaryStatisticsAreaType"><xs:annotation><xs:documentation>The SummaryStatisticsAreaType defines a summary of a single area statistical value.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="SummaryStatisticsType"><xs:attribute name="areaUnit" type="xs:token"><xs:annotation><xs:documentation>The optional areaUnit attribute defines the unit name for the SummaryStatisticsAreaType.</xs:documentation></xs:annotation></xs:attribute></xs:extension></xs:complexContent></xs:complexType>
<xs:attribute name="areaUnit" type="xs:token"><xs:annotation><xs:documentation>The optional areaUnit attribute defines the unit name for the SummaryStatisticsAreaType.</xs:documentation></xs:annotation></xs:attribute>