# BR-CO-26 Seller must be identifiable by an identifier, legal registration ID, or VAT ID

To allow the buyer to identify the supplier automatically, at least one of Seller identifier (BT-29), Seller legal registration identifier (BT-30) or Seller VAT identifier (BT-31) must be present.

## Problem Description

> **ERROR**: EN 16931 calculation and consistency rule (BR-CO)

> **Where BR-CO-26 is checked**
>
> A validator checks an e-invoice in stages. First the file must be well-formed XML that matches the UBL or CII schema (XSD). Then the EN 16931 business rules run: BR core rules, BR-CO calculations, BR-CL code lists, BR-DEC decimals and VAT-category rules such as BR-S. Next come the national or network rules that BT-24 selects: BR-DE for XRechnung, PEPPOL-EN16931 for Peppol BIS Billing. The report lists every failed rule: one error means reject, warnings alone still mean accept. BR-CO-26: EN 16931 calculation rule, reported in the stage “EN 16931 rules”.

## Context and Significance

Buyer systems match the supplier automatically by these identifiers. If none is present, the invoice goes to manual processing and payment is delayed. A tax registration number in BT-32 (for example a German Steuernummer) does not count for this rule. The error typically affects small businesses without a VAT identifier that state only their tax number and no register number or other seller identifier.

## Solution

Populate at least one of BT-29, BT-30 or BT-31 for the seller. UBL: cac:PartyIdentification/cbc:ID (BT-29), cac:PartyLegalEntity/cbc:CompanyID (BT-30), cac:PartyTaxScheme/cbc:CompanyID with cac:TaxScheme/cbc:ID = VAT (BT-31); CII: ram:ID or ram:GlobalID, ram:SpecifiedLegalOrganization/ram:ID, ram:SpecifiedTaxRegistration/ram:ID with schemeID="VA".

- [Validate XML](/en/xrechnung-validator?source=resource&rule=BR-CO-26)
- [Convert invoice](/en/pdf-to-xrechnung?source=resource&rule=BR-CO-26)

## Valid Examples

- BT-31 = "DE123456789"
- BT-30 = "HRB 12345"
- BT-29 = "4012345000092" with schemeID="0088" (GLN)

## Invalid Examples

- Seller party with name and address only, BT-29/BT-30/BT-31 all absent
- BT-31 omitted because the seller has no VAT identifier, while BT-29 and BT-30 are also missing
- Only a tax registration number in BT-32 (cac:PartyTaxScheme with a scheme other than VAT), no BT-29, BT-30 or BT-31

## Affected Fields

- [BT-29 Seller identifier](/resources/xrechnung/bt-29-seller-identifier)
- [BT-30 Seller legal registration identifier](/resources/xrechnung/bt-30-seller-legal-registration-identifier)
- [BT-31 Seller VAT identifier](/resources/xrechnung/bt-31-seller-vat-identifier)

## Related Rules

- [BR-CO-9](/resources/kosit-validator-errors/br-co-9-vat-identifier-must-have-iso-country-prefix)
- [BR-CL-10](/resources/kosit-validator-errors/br-cl-10-iso-6523-any-identifier-scheme)
- [BR-CL-11](/resources/kosit-validator-errors/br-cl-11-iso-6523-registration-scheme)

## Validation Tools

- [Official KoSIT Validator](https://github.com/itplr-kosit/validator): Run the official KoSIT validator with the XRechnung configuration to check EN 16931 and XRechnung rules.
- [Convert PDF to XRechnung](/pdf-to-xrechnung): Find validation errors during conversion and review each correction.

## Related Resources

- [Official Specification](https://docs.peppol.eu/poacc/billing/3.0/rules/ubl-tc434/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)
- [All validation errors](/resources/kosit-validator-errors): Overview of the EN 16931, XRechnung and Peppol validation rules explained here.