The required id attribute is the QIF id of the measurement resource, used for referencing.
Source
<xs:complexType name="DetachableSensorBaseType" abstract="true"><xs:annotation><xs:documentation>The abstract DetachableSensorBaseType is the parent type of more specific types of detachable sensor.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="SensorType"/></xs:complexContent></xs:complexType>