The required id attribute is the QIF id of the measurement resource, used for referencing.
Source
<xs:complexType name="MicrometerDigitalType"><xs:annotation><xs:documentation>The MicrometerDigitalType defines a micrometer measurement device with electronic digital readout.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="MicrometerType"/></xs:complexContent></xs:complexType>