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