- ZUGFeRD (FeRD): https://www.ferd-net.de/
- Factur-X (FNFE-MPE): https://fnfe-mpe.org/factur-x/
- XRechnung (KoSIT): https://xeinkauf.de/xrechnung/
- EN 16931 code lists and validation artefacts (European Commission): https://ec.europa.eu/digital-building-blocks/sites/display/DIGITAL/Registry+of+supporting+artefacts+to+implement+EN16931
- UBL syntax (Peppol BIS Billing 3.0): https://docs.peppol.eu/poacc/billing/3.0/syntax/ubl-invoice/
Envisia.InvoiceXml is based on ZUGFeRD-csharp (https://github.com/stephanstapel/ZUGFeRD-csharp) by Stephan Stapel and its numerous contributors, who built and maintained the library since 2013.