The eFFF standard
The eFFF standard is derived from the UBL standard. This eFFF standard was created as a collaboration between various Belgian software houses, especially of accounting software. That is precisely why this eFFF standard is only used in Belgium. Its success is declining in favor of the PEPPOL standard, which is a standard for the entire European Union.
The eFFF standard is more simple than the PEPPOL standard, because certain values must be entered in it, while they do not even occur in an eFFF file. That is more than enough to exchange invoices.