EN 16931 and UBL: the data a structured invoice must carry

A structured e-invoice is a data file, not a picture of an invoice. Three layers define it: EN 16931, the European standard that lists the business terms an invoice can carry and the rules between them; UBL 2.1, the XML syntax in which Peppol exchanges those terms; and Peppol BIS Billing 3.0, the Peppol specification that narrows and completes EN 16931 for the network. Since 1 January 2026, Belgian law makes Peppol BIS in its UBL version, sent over Peppol, the default format for in-scope B2B invoices. You never write the XML: you enter correct business data, and your invoicing software turns it into a valid file.

This reference is for business owners, finance staff and accountants who want to know what the file contains and why a send can be refused, and for developers who map data to it. It covers the data model and field mapping. For the Peppol profile and document types, see Peppol BIS Billing; to fix a specific error, Peppol validation errors; for the wording the law requires, Belgian invoice legal mentions.

Three layers of one invoice: EN 16931, UBL and Peppol BIS

Business term (BT)

A numbered data element of EN 16931, such as BT-1 (invoice number) or BT-48 (buyer VAT identifier). Groups are numbered BG and business rules BR. Validation messages and software documentation use these numbers.

What the law requires. Under art. 13ter of Royal Decree no. 1 (in force since 1 January 2026), a taxable person who must issue a structured invoice draws it up in line with the European standard, in Peppol BIS (UBL version), and sends it over the Peppol network. Another format or channel is allowed only by agreement between the parties, and it must still comply with the European standard and the list of syntaxes under Directive 2014/55/EU. FPS Finance recommends putting such an agreement in writing (FAQ Q13). Even then, art. 13quater requires you to have the technical means to issue and receive Peppol BIS invoices. Who is in scope: Belgian B2B e-invoicing obligation.

The structured file is the invoice; the PDF is a copy

For in-scope B2B transactions, the structured e-invoice has been the only legally required invoice since 1 January 2026. A PDF copy is optional, and a customer cannot reject an invoice because no PDF is attached (FPS Finance B2B FAQ, Q11 and Q12). No rule says the XML "wins" over a PDF that shows something else: if the two disagree, the invoice is wrong. Correct it with a credit note and a new invoice, not with a new PDF. More in PDF versus structured invoice.

Which data the invoice carries, and where OrdoGrid takes it from

The tables below list the business terms in a Belgian domestic invoice sent from OrdoGrid. "Mandatory" means required by EN 16931 or Peppol BIS; what VAT law requires follows in the next section. Labels on OrdoGrid's invoicing screens are French in every interface language; translations are in brackets.

Document and parties

Payment

Credit notes carry no payment instructions.

Lines, VAT breakdown and totals

Who requires what: VAT law, Peppol BIS and OrdoGrid

VAT law decides what an invoice must say; Peppol BIS decides how the file is structured; OrdoGrid fills both from the same fields.

VAT categories: S, Z, E, AE, K, G and O

Each line and each VAT breakdown carries a category code from the UNCL5305 code list. The category follows from the VAT treatment of the supply, which you decide with your accountant; VAT regimes on Belgian invoices explains the treatments. In OrdoGrid you choose the treatment under Options de TVA, and OrdoGrid sets the category.

Points to watch:

• One regime per document. In OrdoGrid, any regime other than Standard sets every line to 0%, and the editor refuses to save or send an invoice or credit note under Standard that mixes 0% and other rates. If one supply is reverse-charged and another taxed, issue two invoices.
• Extended VAT clauses. FPS Finance asks for extended clauses, such as the reverse-charge mention, as a document note (BT-22) and in the exemption reason (BT-120) (FAQ Q11). A customer should not reject an invoice that says "reverse charge" without the detailed mention, but without it the clause shifting liability to the customer cannot take effect, which mainly affects the supplier (FAQ Q27). OrdoGrid sends the text it prints in BT-120 and in the document note; under Régime cocontractant that is the full art. 20 §3 mention, translated on English and German documents.
• Franchise wording. Under Régime de la franchise de taxe, OrdoGrid prints the wording required since 1 January 2025, "Régime particulier de la franchise de taxe" or "Bijzondere vrijstellingsregeling van belasting", on French and Dutch documents. No official English or German wording exists: English and German documents carry a translation, so choose French or Dutch as Langue des documents (document language) if you want the official text. A business whose operating seat is in the Dutch-language area must use Dutch anyway.
• Intra-EU supplies of goods. Rules BR-IC-11 and BR-IC-12 (fatal) also require the actual delivery date or invoicing period and the deliver-to country. OrdoGrid always sends both under Livraison intracommunautaire (see BT-72 and BT-80 above), so enter the actual date in Delivery / service date when it differs from the invoice date. A credit note for an invoice issued outside OrdoGrid is refused until you fill in that date. You can send these invoices and credit notes over Peppol when OrdoGrid finds the customer there from its VAT number; cross-border invoices are outside the Belgian obligation, so email remains possible.
• Out of scope (O). Category O cannot be combined with other categories on one invoice (BR-O-11). As a temporary workaround, FPS Finance accepts out-of-scope lines under E with a reason text (FAQ Q8). OrdoGrid does not offer O: ask your accountant how to invoice such lines.

Worked example: a two-rate invoice, term by term

Agrifourniture Hesbaye SRL in Hannut (enterprise number 0451.823.624) sells to Jardins Verhaegen SRL in Wavre (enterprise number 1006.239.101, a recent number starting with 1). Jardins Verhaegen requires a purchase order on every invoice, so its client record has Exiger un bon de commande pour les factures (require a purchase order on invoices) ticked. Invoice F-2026-0187 is dated 5 October 2026, the goods are delivered the same day, and the due date is 4 November 2026.

Fertilisers are taxed at 6% unless mixed with plant-protection products, and plant-protection products moved from 12% to 21% on 1 March 2026 (Royal Decree of 14 February 2026). Since 1 January 2026, structured invoices issued under the B2B obligation follow the rounding rules of EN 16931 (Royal Decree no. 8, art. 1, fourth paragraph), which calculate VAT per category and rate:

• 6%: €736.00 × 6% = €44.16.
• 21%: €257.40 × 21% = €54.054, rounded to €54.05.
• Total excluding VAT €993.40; total VAT €98.21; total including VAT €1,091.61.

Calculating VAT per can instead (€42.90 × 21% = €9.009, rounded to €9.01, × 6 = €54.06) would give one cent more. The standard calculates VAT on the total per rate, and so does OrdoGrid.

A simplified excerpt of the UBL for the VAT breakdown and totals:

<cbc:BuyerReference>4500127</cbc:BuyerReference>
<cac:TaxTotal>
  <cbc:TaxAmount currencyID="EUR">98.21</cbc:TaxAmount>
  <cac:TaxSubtotal>
    <cbc:TaxableAmount currencyID="EUR">736.00</cbc:TaxableAmount>
    <cbc:TaxAmount currencyID="EUR">44.16</cbc:TaxAmount>
    <cac:TaxCategory><cbc:ID>S</cbc:ID><cbc:Percent>6</cbc:Percent></cac:TaxCategory>
  </cac:TaxSubtotal>
  <cac:TaxSubtotal>
    <cbc:TaxableAmount currencyID="EUR">257.40</cbc:TaxableAmount>
    <cbc:TaxAmount currencyID="EUR">54.05</cbc:TaxAmount>
    <cac:TaxCategory><cbc:ID>S</cbc:ID><cbc:Percent>21</cbc:Percent></cac:TaxCategory>
  </cac:TaxSubtotal>
</cac:TaxTotal>
<cac:LegalMonetaryTotal>
  <cbc:LineExtensionAmount currencyID="EUR">993.40</cbc:LineExtensionAmount>
  <cbc:TaxExclusiveAmount currencyID="EUR">993.40</cbc:TaxExclusiveAmount>
  <cbc:TaxInclusiveAmount currencyID="EUR">1091.61</cbc:TaxInclusiveAmount>
  <cbc:PrepaidAmount currencyID="EUR">0.00</cbc:PrepaidAmount>
  <cbc:PayableAmount currencyID="EUR">1091.61</cbc:PayableAmount>
</cac:LegalMonetaryTotal>

The totals must reconcile exactly after rounding to the cent: the sum of line net amounts equals BT-106 (BR-CO-10), BT-109 equals BT-106 minus document allowances plus charges (BR-CO-13), BT-110 is the sum of the breakdown VAT amounts (BR-CO-14), BT-112 equals BT-109 plus BT-110 (BR-CO-15), and BT-115 equals BT-112 minus BT-113 plus BT-114 (BR-CO-16). Validation is looser elsewhere: PEPPOL-EN16931-R120 (line net amount) and R040 (allowance or charge percentage) allow 0.02, and BR-CO-17 accepts a category VAT amount that differs by less than 1.00 from taxable amount × rate. Belgian law still requires the VAT per rate rounded to the cent, as calculated above. For a rounding line from VAT-inclusive prices, see VAT-inclusive prices, discounts and rounding.

Variant: construction reverse charge (AE)

Menuiserie Lambert SRL places ten windows at €450.00 each in an office building for Horizon Construct SRL, which files periodic VAT returns. With Régime cocontractant, the file carries one VAT breakdown: category AE, 0%, taxable amount €4,500.00, VAT €0.00, reason code VATEX-EU-AE and, as text, the cocontractant mention OrdoGrid prints on the PDF, which also opens the document note (BT-22). BT-110 is €0.00, and BT-112 and BT-115 are €4,500.00. Horizon Construct declares the VAT itself in its return: €945.00 at the standard 21% rate. The seller's and the buyer's VAT numbers, which rule BR-AE-02 requires (for the buyer, the VAT number or a legal registration identifier), are both in the file, and the mention OrdoGrid prints is the full art. 20 §3 text.

Check the data before you send from OrdoGrid

1. In Profil entreprise (company profile), complete Nom de l'entreprise, Numéro de TVA (Optionnel), which must start with BE for Peppol, Adresse, Code postal, Ville, Pays and, for payment details in the file, IBAN. Only owners and administrators can edit this page.
2. In Base Clients (client list), open the customer with Ajouter un client (add a client) or Modifier le client (edit client). Choose Assujetti à la TVA (VAT-registered), enter N° de TVA (VAT number) and let the lookup fill name and address (badge Registre belge (BCE)), and check Pays. If the customer issues purchase orders, tick Exiger un bon de commande pour les factures.
3. In the editor (Nouvelle facture, new invoice), check N°, Date and Échéance, then fill each line: Description, Quantité, Unité, Prix Unitaire, TVA.
4. Under Options de TVA, choose the regime. Any regime other than Standard sets all lines to 0%.
5. Under Documents et Références (documents and references), fill in N° bon de commande: it is shown, and required, when the client record asks for a PO; for any other client, open it with Ajouter un N° bon de commande (add a purchase-order number). In the payment block, keep the Communication OrdoGrid fills in (a structured communication built from the invoice number), and add in Notes any legal wording OrdoGrid does not print.
6. Click Aperçu du PDF (PDF preview), then Envoyer (send). Make sure Envoi électronique via Peppol (send electronically via Peppol) is ticked and click Sauvegarder & Envoyer (save and send).
7. Before delivery, OrdoGrid builds the UBL and checks it against the UBL 2.1 schema, the EN 16931 rules (validation artefacts 1.3.16) and the Peppol BIS Billing 3.0.21 rules. If the schema check or a fatal rule fails, nothing is sent (warnings do not block): the result shows an error on the Peppol line and the message Des erreurs sont survenues. (errors occurred). Correct the data and send again.
8. In Peppol → Outbox, Traité (processed) means the network delivered the file. It does not mean the customer accepted it or that the data was right.

Once an invoice has been prepared or sent on Peppol, its amounts are locked; correct them with a credit note (correct or credit an invoice).

Edge cases and common mistakes

Evidence to keep

• The XML as sent. In Peppol → Outbox, use the XML download, or select rows for a ZIP with an xml and a pdf folder. OrdoGrid archives the exact XML and PDF it sent, with the validation versions and SHA-256 fingerprints.
• The PDF as issued: Original document in the Settlement panel stays unchanged.
• Agreements: any written agreement to use another format or channel than Peppol BIS (art. 13ter), and the customer's instructions on references or PO numbers.

VAT law requires invoices to be kept for 7 years from 1 January of the year after issue (art. 60 VAT Code, as amended by the Law of 18 December 2025); documents linked to the revision of VAT on immovable property must be kept longer. FPS Finance strongly recommends keeping the original XML. OrdoGrid is not a legal archiving service, so keep your own copies: see invoice archiving.

FAQ

What is EN 16931?

The European standard for electronic invoicing. Part 1 defines the business terms a core invoice can carry, such as parties, lines, VAT breakdown and totals, and the rules between them. Peppol BIS Billing 3.0 is built on the 2017 version; the 2026 version published in May 2026 does not change what you must do today.

What is UBL, and is it the only XML format?

UBL 2.1 is one of the two XML syntaxes EN 16931 recognises; the other is UN/CEFACT CII. Peppol BIS Billing requires UBL, and Belgian law names Peppol BIS in its UBL version as the default format. Another syntax is possible only by agreement with your customer.

Is a purchase order number mandatory on a Peppol invoice?

Not under VAT law. Peppol BIS requires a buyer reference or an order reference on every invoice and credit note (rule PEPPOL-EN16931-R003). OrdoGrid always fills the buyer reference: with the PO number if you entered one, otherwise the communication or the invoice number.

Are an IBAN and a due date mandatory?

Neither is a VAT mention, and Peppol BIS 3.0.21 requires neither: rule BR-CO-25 on due dates was removed in that release (May 2026). OrdoGrid always sends the due date. When your IBAN is filled in, it also sends the IBAN and the communication (generated from the invoice number if you left Communication empty), which lets the customer's software prepare the payment.

If the PDF and the XML differ, which one counts?

Neither prevails by rule. For in-scope B2B invoices the structured invoice is the legal invoice, but a contradictory PDF means the invoice contains an error. Issue a credit note and a new invoice.

Do I have to write or edit the XML?

No. OrdoGrid builds the UBL from your profile, client record and invoice, and validates it before delivery. When validation fails, correct the data in OrdoGrid, not the XML.