# BT-31 Seller VAT Identifier

The seller's VAT identifier (also known as VAT registration number). For German companies, this follows the format DE + 9 digits.

> BT-31 (Seller VAT identifier) is the supplier's VAT registration number, prefixed by the ISO 3166-1 country code. Conditional in EN 16931: required whenever the invoice has any VAT amount (BR-CO-9), unless BT-32 (tax registration identifier) is supplied instead. Example: DE123456789.
>
> BT-31 · EN 16931 / XRechnung 3.x

## BT Reference Card

Canonical EN 16931 / XRechnung facts for this business term — citable from this anchor.

- **BT code**: BT-31
- **Name**: Seller VAT Identifier
- **Cardinality**: Conditional (C)
- **Business group**: BG-4 SELLER
- **Data type**: Identifier
- **Example value**: DE123456789
- **Common error**: BR-CO-9: At least one of BT-31 (Seller VAT identifier) or BT-32 (Seller tax registration identifier) must be present whenever the invoice contains VAT (BG-23).

## Quick answer

The seller's VAT identifier (also known as VAT registration number). For German companies, this follows the format DE + 9 digits.

> **Seller details**: Who issued the invoice and how the recipient can identify that business reliably.

## In plain language

The seller's VAT identifier (also known as VAT registration number). For German companies, this follows the format DE + 9 digits.

The seller's VAT identifier (also known as VAT registration number). For German companies, this follows the format DE + 9 digits.

Critical for VAT compliance and cross-border trade within the EU. Required when the seller is subject to VAT.

## What you usually enter here

Country code (2 letters) + national VAT number

Typical valid values include: DE123456789, AT12345678, FR12345678901, NL123456789B01

Required if the seller is registered for VAT according to EU VAT Directive 2006/112/EC

## What goes wrong often

Avoid values like: 123456789, DE12345678, DE1234567890, DE 123456789

Relevant rule codes: BR-CO-26, BR-DE-16

## Examples in context

Critical for VAT compliance and cross-border trade within the EU. Required when the seller is subject to VAT.

Required if the seller is registered for VAT according to EU VAT Directive 2006/112/EC

## Valid Examples

- DE123456789
- AT12345678
- FR12345678901
- NL123456789B01
- IT12345678901

## Invalid Examples

- 123456789
- DE12345678
- DE1234567890
- DE 123456789
- de123456789

## Related Fields

Critical for VAT compliance and cross-border trade within the EU. Required when the seller is subject to VAT.

- [BT-27 Seller Name](/resources/xrechnung/bt-27-seller-name)
- [BT-30 Seller legal registration identifier](/resources/xrechnung/bt-30-seller-legal-registration-identifier)
- [BT-32 Seller tax registration identifier](/resources/xrechnung/bt-32-seller-tax-registration-identifier)
- [BT-48 Buyer VAT Identifier](/resources/xrechnung/bt-48-buyer-vat-identifier)

## 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-31

### Data Type:
identifier

### Cardinality:
0..1

### Requirement:
Optional

### Max Length:
30 characters

### Since Version:
Conditional (required if subject to VAT)

## XML Implementation

### UBL 2.1 Syntax
```
/Invoice/cac:AccountingSupplierParty/cac:Party/cac:PartyTaxScheme/cbc:CompanyID
```

### CII (UN/CEFACT) Syntax
```
/CrossIndustryInvoice/SupplyChainTradeTransaction/ApplicableHeaderTradeAgreement/ram:SellerTradeParty/ram:SpecifiedTaxRegistration/ram:ID[@schemeID="VA"]
```

## Validation Rules

### BR-CO-26
BR-CO-26

### BR-DE-16
BR-DE-16

## Why the standard cares about this field

### Business Context
Critical for VAT compliance and cross-border trade within the EU. Required when the seller is subject to VAT.

### Legal Requirement
Required if the seller is registered for VAT according to EU VAT Directive 2006/112/EC

## FAQ

### What is the format for German VAT identifiers?
German VAT identifiers follow the format "DE" followed by exactly 9 digits (e.g., DE123456789). No spaces or special characters are allowed.

### Is the VAT identifier mandatory for all sellers?
No, it is only required if the seller is registered for VAT. Small businesses below the VAT threshold or VAT-exempt entities may not have a VAT identifier.

### How can I validate a VAT identifier?
Use the EU VIES system (ec.europa.eu/taxation_customs/vies/) to validate VAT identifiers. Each country has specific format requirements.

### What about other EU countries VAT formats?
Each EU country has its own VAT number format: AT (8 digits), FR (11 characters), NL (12 characters), IT (11 digits), etc. Always include the country code prefix.

## Official sources and tools

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