# XML Schema (XSD)

A formal definition of allowed XML structure and data types.

## Definition

A formal definition of allowed XML structure and data types.

## Explanation

Used for syntax validation (XSD validation).

Business rules go beyond XSD and check semantic constraints.

## Related resources

- [UBL guide](/resources/compliance/ubl-universal-business-language)

## Related terms

- [Validation](/resources/glossary/validation): Process of checking an e-invoice against syntax rules and business rules.
- [XML](/resources/glossary/xml): Text-based markup format used to represent structured data in many e-invoice syntaxes.
- [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.