The required n attribute is the number of Id elements in this array.
Source
<xs:element name="WeldCharacteristicItemIds" type="ArrayReferenceType"><xs:annotation><xs:documentation>The WeldCharacteristicItemIds element is a list of the QIF ids of the weld characteristic items.</xs:documentation></xs:annotation></xs:element>
The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="WeldCompoundCharacteristicItemType"><xs:annotation><xs:documentation>The WeldCompoundCharacteristicItemType defines a compound weld characteristic item.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="WeldCharacteristicItemBaseType"><xs:sequence><xs:element name="WeldCharacteristicItemIds" type="ArrayReferenceType"><xs:annotation><xs:documentation>The WeldCharacteristicItemIds element is a list of the QIF ids of the weld characteristic items.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:extension></xs:complexContent></xs:complexType>