The id attribute is the QIF id of the action method, used for referencing.
Source
<xs:complexType name="MicroscopeMeasureFeatureMethodType"><xs:annotation><xs:documentation>The MicroscopeMeasureFeatureMethodType is the metrology method in which a microscope is used for measuring the feature.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="MeasureFeatureMethodBaseType"/></xs:complexContent></xs:complexType>