The required n attribute gives the number of carriages in the list.
Source
<xs:element name="Carriages" type="CarriagesType"><xs:annotation><xs:documentation>The Carriages element is a list of carriages.</xs:documentation></xs:annotation></xs:element>
The required id attribute is the QIF id of the measurement resource, used for referencing.
Source
<xs:complexType name="MultipleCarriageCartesianCMMType"><xs:annotation><xs:documentation>The MultipleCarriageCartesianCMMType defines a Cartesian CMM with two or more carriages.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="CartesianCMMType"><xs:sequence><xs:element name="Carriages" type="CarriagesType"><xs:annotation><xs:documentation>The Carriages element is a list of carriages.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:extension></xs:complexContent></xs:complexType>