Additional document reference are used for different document references, required document, provided document. To reference several types, the additional document should be repeated for each type.

cac urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2
Data type
Child elements
Card Name Description
1..1 cbc:ID

Document identifier
An identifier that is specified by the buyer and used as a reference number for all documents in the procurement process. It is also known as procurement project identifier, procurement reference number or contract folder identifier.

0..n cbc:XPath

Reference to an cac:Attachment
An xpath expression to reference an cac:Attachment where questions can be asked or refer a file within the ASiC-e.