You can use the AdditionalDocumentReference element to reference another. For example, you can add a URL that references another document.

Elements

Element	Description
`Invoice/AdditionalDocumentReference/ID`	The ID of the referenced additional document.
`Invoice/AdditionalDocumentReference/DocumentDescription`	A description of the referenced additional document.
`Invoice/AdditionalDocumentReference/Attachment/ExternalReference/URI`	The Unique Resource Identifier for the referenced additional document.

Country Specific Configurations

Greece: Additional Document Reference

Example

<ubl:Invoice>
    <!-- Code omitted for clarity -->
    <cac:AdditionalDocumentReference>
	     <cbc:ID/>
		   <cbc:DocumentDescription>VRBL:GR:INVOICEURL</cbc:DocumentDescription>
		   <cac:Attachment>
			  <cac:ExternalReference>
				  <cbc:URI>http://www.example.com/index.html</cbc:URI>
			  </cac:ExternalReference>
		   </cac:Attachment>
	  </cac:AdditionalDocumentReference>
    <!-- Code omitted for clarity -->
</ubl:Invoice>