The id attribute is the QIF id of the action method, used for referencing.
Source
<xs:complexType name="ProfileProjectorMeasureFeatureMethodType"><xs:annotation><xs:documentation>The ProfileProjectorMeasureFeatureMethodType is the metrology method in which an optical comparator (shadowgraph) is used for measuring the feature.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="MeasureFeatureMethodBaseType"/></xs:complexContent></xs:complexType>