The required id attribute is the QIF id of the measurement resource, used for referencing.
Source
<xs:complexType name="ToolBaseType" abstract="true"><xs:annotation><xs:documentation>The abstract ToolBaseType defines the base type for a measurement device that may be mounted on a CMM.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="MeasurementResourceBaseType"/></xs:complexContent></xs:complexType>