# BT-122 Supporting Document Reference

An identifier for a supporting document referenced from the invoice.

## Quick answer

> **Taxes and totals**: VAT treatment, tax breakdowns, and document totals that validators check closely.

- [Validate XML](/en/xrechnung-validator?source=resource&bt=BT-122)
- [Convert invoice](/en/pdf-to-xrechnung?source=resource&bt=BT-122)

## In plain language

An identifier for a supporting document referenced from the invoice.

An identifier for a supporting document referenced from the invoice.

Links the invoice to evidence such as contracts, delivery notes, timesheets, or other attachments.

## What you usually enter here

Document identifier

Typical valid values include: DELIVERY-NOTE-1001, CONTRACT-2025-07, TIMESHEET-42

Each supporting document group must contain a supporting document reference according to BR-52.

## What goes wrong often

Avoid values like: ,

Relevant rule codes: BR-52

## Examples in context

Links the invoice to evidence such as contracts, delivery notes, timesheets, or other attachments.

Each supporting document group must contain a supporting document reference according to BR-52.

## Valid Examples

- DELIVERY-NOTE-1001
- CONTRACT-2025-07
- TIMESHEET-42

## Related Fields

Links the invoice to evidence such as contracts, delivery notes, timesheets, or other attachments.

- [BT-123 Supporting Document Description](/resources/xrechnung/bt-123-supporting-document-description)
- [BT-124 External Document Location](/resources/xrechnung/bt-124-external-document-location)
- [BT-125 Attached Document](/resources/xrechnung/bt-125-attached-document)

## XML mapping and field facts

Open the technical reference if you need XML paths, validator rule codes, legal wording, or schema-specific mapping details.

### Field ID:
BT-122

### Data Type:
identifier

### Cardinality:
1..1

### Requirement:
Mandatory

### Since Version:
Mandatory when BG-24 supporting document is provided

## XML Implementation

### UBL 2.1 Syntax
```
/Invoice/cac:AdditionalDocumentReference/cbc:ID
```

### CII (UN/CEFACT) Syntax
```
/CrossIndustryInvoice/SupplyChainTradeTransaction/ApplicableHeaderTradeAgreement/ram:AdditionalReferencedDocument/ram:IssuerAssignedID
```

## Validation Rules

### BR-52
BR-52

## Check whether your XRechnung sets BT-122 correctly

Upload the XML file to the free validator. It applies the KoSIT rules for XRechnung 3.0 and names each finding with its rule ID, for this field for example BR-52. Creating a new invoice instead? The converter checks the invoice against the same rules and shows the findings before download.

- [Validate XML](/en/xrechnung-validator)
- [Convert invoice](/en/pdf-to-xrechnung)

Free, no account needed. XML file up to 2 MB.

## Why the standard cares about this field

### Business Context
Links the invoice to evidence such as contracts, delivery notes, timesheets, or other attachments.

### Legal Requirement
Each supporting document group must contain a supporting document reference according to BR-52.

## Official sources and tools

- [KoSIT Validator](https://erechnungsvalidator.service-bw.de/)
- [XRechnung Standard](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)