# BT-122 — Supporting document reference

> https://billhorse.com/en/terms/bt-122/

Identifier of a document that substantiates the invoice (e.g. timesheet, delivery note). Mandatory within group BG-24 whenever an attachment is supplied.

## Where does it live in the XML?

- UBL: `cac:AdditionalDocumentReference/cbc:ID`
- CII: `ram:ApplicableHeaderTradeAgreement/ram:AdditionalReferencedDocument/ram:IssuerAssignedID (TypeCode 916)`