XML (Extensible Markup Language)

What is XML?

XML stands for Extensible Markup Language. It is a universal coding language used to structure and transport data between IT systems. Developed by the World Wide Web Consortium (W3C) in the late 1990s, it is now a cornerstone of digital data exchange worldwide. Unlike HTML, which tells a browser how to display content, XML describes what the data is. For example, a field can be tagged as InvoiceTotal, and any machine will know exactly what that number represents without having to guess.

XML in the world of invoicing

When discussing e-invoices, XML is the core format. International standards like Peppol BIS Billing 3.0 are entirely XML-based. An XML invoice is essentially a text file with "tags" that define every single data point: supplier name, VAT number, line items, tax amounts, and more. Because the structure is predictable, the recipient's system can automatically import and post the invoice without manual data entry errors.

The difference between XML and PDF

A PDF is designed for human eyes: it shows a nice layout with logos, fonts, and colors. However, to a machine, a PDF is practically an image—the text and numbers have no fixed structure. A system must use OCR or scanning to "guess" the data, which leads to errors. XML flips this around. The layout is irrelevant; instead, every data point is uniquely labeled. This is why legislation globally is moving toward XML-based invoices—it makes accounting faster and more reliable.

Common XML variants

There are several XML-based standards depending on the country and industry: Peppol BIS: The common European format gaining ground globally. XRechnung: The specific German standard for B2G invoicing. EHF: Norway's national XML-based format. ZUGFeRD / Factur-X: Hybrid formats that combine a human-readable PDF with an embedded XML file.

Why XML remains relevant

Despite newer alternatives like JSON, XML remains the gold standard for business documents. This is due to its strict validation (via XML Schemas), heavy support from tax authorities, and mature toolsets. With the EU's ViDA initiative (VAT in the Digital Age) requiring e-invoicing across member states, XML formats will only become more critical in the years to come.