Glossary Term
BT-1

BT-1 Invoice Number

BT-1 is the unique sequential identifier assigned to an invoice by the seller, required in every EN 16931 e-invoice.

Quick Facts

Parent
Invoice root
Data type
Identifier
Mandatory
Yes
Cardinality
1..1

Definition

What is BT-1?

BT-1 (Invoice number) is a mandatory business term in the EN 16931 data model. It uniquely identifies a specific invoice within the seller's numbering system. Every valid e-invoice must have a BT-1 value.

Requirements

  • Must be unique within the seller's invoice numbering sequence

  • Mandatory (cardinality 1..1) — no invoice can omit this field

  • Should be sequential (many countries require gap-free sequences)

  • Data type: Identifier (free text, but typically alphanumeric)

    • In UBL XML

    INV-2025-00001

    The element is a direct child of the Invoice root element.

    In CII XML


      INV-2025-00001

    Numbering Best Practices

    Pattern Example Notes
    -------------------------
    Year prefix 2025-00001 Resets annually, easy to organize
    Sequential 00000001 Simple, but hard to determine year
    Branch prefix BRU-2025-001 Useful for multi-location businesses
    Series code A-2025-001 When using multiple numbering series

    Country-Specific Rules

  • Germany: Invoice numbers must be sequential within each numbering series (§14 UStG). Gaps may trigger audit questions.

  • Italy (SDI): Duplicate invoice numbers within the same year are rejected by SDI.

  • France: Chronological, unbroken sequences required per tax regulation.

    • Common Errors

  • Duplicate numbers — reusing an invoice number causes rejection in SDI and other systems

  • Gaps in sequence — missing numbers may raise audit flags

  • Too long — some receiving systems truncate after 20 characters

  • Special characters — avoid characters that may cause XML encoding issues (&, <, >)

    • XML Examples

    UBL (Peppol, XRechnung)

    <cbc:ID>INV-2025-00001</cbc:ID>

    CII (ZUGFeRD, Factur-X)

    <ram:ID>INV-2025-00001</ram:ID>

    Related Content

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