EN 16931 is the semantic standard for "what data an invoice should contain." Peppol is a network and specification for transmitting e-invoices (Belgium's mandate uses Peppol). Factur-X (called ZUGFeRD in Germany) is a file format: EN 16931-compliant XML embedded in a PDF/A-3 file, readable by both humans and machines in a single file.
Full details and tax ID checklist: https://patisco.com/zh/eu-check/e-invoicing Check which EU regulations apply to you by tax ID: https://patisco.com/zh/eu-check