The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="FormCharacteristicMeasurementBaseType" abstract="true"><xs:annotation><xs:documentation>The FormCharacteristicMeasurementBaseType is the abstract base type that defines the results of a measured form characteristic evaluation.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="GeometricCharacteristicMeasurementBaseType"/></xs:complexContent></xs:complexType>