# XML

Text-based markup format used to represent structured data in many e-invoice syntaxes.

## Definition

Text-based markup format used to represent structured data in many e-invoice syntaxes.

## Explanation

UBL and CII are XML-based invoice syntaxes.

XML must be well-formed and conform to schemas/business rules.

## Related resources

- [Convert invoices](/convert)

## Related terms

- [XML Schema (XSD)](/resources/glossary/schema): A formal definition of allowed XML structure and data types.
- [Validation](/resources/glossary/validation): Process of checking an e-invoice against syntax rules and business rules.
- [UBL](/resources/glossary/ubl): Universal Business Language: an XML library for standard business documents.
- [CII](/resources/glossary/cii): UN/CEFACT Cross Industry Invoice: an XML syntax used for e-invoices.