The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="LinearCharacteristicItemBaseType" abstract="true"><xs:annotation><xs:documentation>The LinearCharacteristicItemBaseType is the abstract base type that defines a linear characteristic item.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="DimensionalCharacteristicItemBaseType"/></xs:complexContent></xs:complexType>