Key takeaways
- UBL 2.1 (Universal Business Language) is an open OASIS standard that describes, in XML, the structure of an e-invoice.
- A UBL invoice is not a PDF: every value is tagged and read automatically by software, with no re-keying.
- EN 16931 defines what an invoice must contain; UBL 2.1 is one of the two XML syntaxes that express it.
- Peppol BIS Billing 3.0 is built on UBL 2.1: the format mandated for B2B in Belgium is technically a UBL file.
UBL 2.1, the language of structured e-invoices
Since 1 January 2026, structured e-invoicing has been mandatory between businesses in Belgium. Behind that mandate sits a file that is often mentioned but rarely described: the UBL 2.1 format. It is what turns an invoice into data that software understands, rather than a flat image.
UBL stands for Universal Business Language. It is an open standard published by OASIS (Organization for the Advancement of Structured Information Standards) that defines the structure of business documents in XML. Version 2.1 was approved as an OASIS Standard in 2013 and later adopted internationally as ISO/IEC 19845:2015.
This article describes what a UBL invoice actually is: where the standard comes from, how the file is organised, and how it fits together with the European standard EN 16931 and the Peppol BIS profile. For the regulatory side of the mandate, read Peppol in Belgium: the B2B mandate on 1 January 2026.
Where the UBL 2.1 format comes from
UBL is neither recent nor specific to Belgium. The standard has been developed since the early 2000s by OASIS, an international standards consortium. Its aim is to offer a common, royalty-free vocabulary for electronic business exchanges, from the order to the despatch advice and the invoice.
Version 2.1 covers far more than the invoice: it defines the structure of dozens of business document types. For invoicing, two documents matter: the invoice (Invoice) and the credit note (CreditNote). These are the ones that European e-invoicing reuses.
The value of an open standard is its neutrality: it belongs to no single software vendor, which lets different tools exchange invoices without a prior agreement. That property is what made UBL 2.1 one of the technical pillars of e-invoicing in Europe.
The structure of a UBL 2.1 invoice
A UBL invoice is an XML file organised into blocks. Everything sits inside a root Invoice element, within which the information is grouped by nature. Two families of tags structure the whole: basic components, which carry a single value (an identifier, a date), and aggregate components, which group several related values (a seller's full address, an invoice line).
In practice you will never write this file by hand: your software produces it from the data you enter. But knowing its main blocks helps explain why a structured invoice is processed without re-keying.
The main blocks of a UBL 2.1 invoice
Document header
Invoice number, issue date, due date, document type and currency.
The seller
Name, address and identifiers of the supplier, including the VAT number.
The buyer
Name, address and identifiers of the receiving customer.
The invoice lines
For each item or service: description, quantity, unit price and VAT rate.
The VAT totals
The breakdown of tax by applicable rate.
The monetary totals
The net total, the VAT total and the amount payable.
This layout explains the concrete gain of a structured invoice: the recipient's software knows exactly where to read the amount payable or the due date, because those values always sit in the same place in the file. No human reading, no interpretation, no copy-paste error.
UBL 2.1, EN 16931 and Peppol BIS: how they fit together
This is where the three terms people tend to confuse each find their place. They are not competitors; they stack on top of one another.
The European standard EN 16931 defines the semantic model of the invoice: the list of information an e-invoice must or may contain, independently of how it is written. It does not in itself prescribe a language; it allows two syntaxes to express it: UBL 2.1 and UN/CEFACT CII.
UBL 2.1 is therefore one of those two syntaxes: the concrete XML grammar used to write an invoice that follows the model. Peppol BIS Billing 3.0, finally, is a core invoice usage specification (CIUS) of EN 16931 that adopts the UBL 2.1 syntax and adds the validation and exchange rules specific to the Peppol network.
UBL 2.1 syntax
machine-readable, not for the eye alone
syntaxes allowed by EN 16931
UBL 2.1 and UN/CEFACT CII
basis of Peppol BIS
Peppol BIS Billing 3.0 (CIUS of EN 16931)
In short: a Peppol BIS invoice is a UBL 2.1 file, which follows the EN 16931 model, and which the Peppol network can transport. Understanding that network is a topic of its own: see Understanding the Peppol network in 5 minutes.
UBL 2.1 versus the PDF
The difference with a PDF is not about image quality, but about the nature of the file. The table below sums up what separates the two.
| PDF sent by email | UBL 2.1 invoice | |
|---|---|---|
| Data read automatically by software | ||
| Processed without manual re-keying | ||
| Compliant with the B2B mandate in Belgium | ||
| Directly readable by the human eye |
The PDF has not disappeared for all that: compliant software generates the UBL 2.1 file for machine-to-machine exchange while also producing a readable version for your records. The topic is covered in E-invoice or PDF: what is the difference.
What it changes for your business
The good news is that you do not have to handle UBL directly. The job of compliant invoicing software is precisely to hide this plumbing: you enter an invoice as usual, the tool produces the UBL 2.1 file, validates it against the Peppol BIS rules and routes it over the Peppol network.
What changes is the requirement on the tool you use. A free-form PDF no longer suffices in B2B; your software must be able to generate, send and receive compliant UBL files. That is the criterion to check first when choosing or updating your solution.
Compliant UBL 2.1 invoices, without touching the XML
YouInv generates your invoices in the Peppol BIS format (UBL 2.1), validates them and sends them over the Peppol network in a few clicks.
Further reading
- Understanding the Peppol network in 5 minutes: the four-corner model and the role of access points.
- Peppol in Belgium: the B2B mandate on 1 January 2026: who is in scope and which format is required.
- Getting ready for e-invoicing in 2026: becoming compliant, step by step.
The reference sources are authoritative: the OASIS UBL 2.1 specification and the Peppol BIS Billing 3.0 documentation.
What is the UBL 2.1 format?
UBL 2.1 (Universal Business Language) is an open OASIS standard that defines, in XML, the structure of business documents such as the invoice and the credit note. It is one of the two syntaxes allowed by the European standard EN 16931 and the basis of the Peppol BIS Billing 3.0 profile.
What is the difference between UBL 2.1 and a PDF?
A PDF is designed for the human eye: software cannot reliably extract its data. A UBL 2.1 invoice is a structured XML file in which every value (amount, VAT, due date) is tagged and read automatically by software, with no re-keying.
Are UBL 2.1 and EN 16931 the same thing?
No. EN 16931 is the European standard that defines the semantic model of the invoice (what information it contains). UBL 2.1 is one of the two XML syntaxes allowed to express that model, the other being UN/CEFACT CII.
How does UBL 2.1 relate to Peppol BIS Billing 3.0?
Peppol BIS Billing 3.0 is a core invoice usage specification (CIUS) of EN 16931 that builds on the UBL 2.1 syntax. A Peppol BIS invoice is therefore a UBL 2.1 file, plus the validation rules specific to the Peppol network.
Do you need to know XML to issue a UBL 2.1 invoice?
No. Compliant invoicing software generates the UBL 2.1 file for you from the data you enter on screen. You never have to write or read the XML by hand.
