The id attribute is the QIF id of the action method, used for referencing.
Source
<xs:complexType name="MeasureFeatureMethodBaseType" abstract="true"><xs:annotation><xs:documentation>The MeasureFeatureMethodBaseType is the abstract base type that defines a feature measurement method.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ActionMethodBaseType"/></xs:complexContent></xs:complexType>