Skip to content

EU e-Invoicing core concepts

Prior to using this library, there are some key concepts about European e-Invoicing you should be aware of.

EN 16931

The European e-Invoicing standard (EN 16931) describes the semantic data model and business rules of an electronic invoice. In other words, this document (specifically part 1) specificies what an invoice is (fields it must/can have) and the possible values those fields can contain.

But that's it: it just gives an abstract definition of an invoice without getting into the technical structure (syntax). We need a format (or formats) for representing electronic invoices as documents (files), and the two specifications allowed by the standard are UBL and CII.

The main purpose of this library is to represent invoices as defined by the European standard and convert them from/to its document representations in UBL or CII.


UBL (Universal Business Language) and CII (UN/CEFACT Cross Industry Invoice) are two XML-based specifications for exchanging business information. They both already existed prior to the European standard and were then chosen as the only supported syntaxes for representing invoice documents that comply with EN 16931.

In theory, they can be used interchangeable with respect to EU e-invoices, although some exceptions may apply depending on additional business rules enforced by a CIUS or an extension.

To determine which field from an European invoice goes to which UBL/CII XML node, there are mappings defined for both formats in parts 2 and 3 of the European standard.

CIUS and extensions

Not all European member states have the same legislation (e.g. tax rates may not be the same across borders) nor all business sectors require the same set of fields to be present in an invoice (e.g. the energy sector may need a document to contain the meter info and meter readings of a client while others don't).

To solve this problem, the European e-Invoicing standard proposes the use of CIUS (Core Invoice Usage Specifications) and Extensions, which are additional specifications made by EU state governments or other stakeholders that add or remove business rules from the EN 16931.

This way, the standard can stay uniform across all European members whilst providing enough flexibility for handling more particular cases, such us the ones mentioned before.

The most popular CIUS is PEPPOL BIS Billing 3.0, a cross-border specification used by multiple countries and international companies from both public and private sectors.