You may expect from an e-invoice standard like EN16931 that it standardizes e-invoices. Not quite! Each member state can define their own extentions to the standard called Core Invoice Usage Specifications (CIUS). Notable examples are the standard "XRechnung" in Germany or "OIUBL" in Denmark, see the EU document eInvoicing Country Factsheets for each Member State & other countries for an overview.
Fortunately, countries cannot arbitrarily change the standard.
First of all, the allowed syntaxes are still only UBL and CII.
Changes to the semantics are also restricted. Countries can, for example, make optional elements mandatory, restrict the number of occurrences of a certain element, or disallow usage of certain codes. See CIUS and Extension - What is allowed for more information.
Both UBL and CII require the presence of a so-called customization ID. The customization ID is an opaque string like this:
urn:cen.eu:en16931:2017#compliant#urn:fdc:peppol.eu:2017:poacc:billing:3.0
It identifies a particular flavour of the e-invoice.
E-Invoice-EU facilitates this by defining aliases for certain formats. For example the alias "XRechnung" for the German extension automatically sets the customization id of the document to this:
urn:cen.eu:en16931:2017#compliant#urn:xeinkauf.de:kosit:xrechnung_3.0
If you want to produce a flavour that is not directly supported by E-Invoice-EU, you can simply select the base format (UBL or CII) and also specify the customization ID yourself like any other field. Explicitly specified customization IDs always override the built-in defaults.
Either way, it is your responsability to fulfill the semantic requirements of every country-specific flavour. E-Invoice-EU only checks that the structure of the invoice data adheres to the stipulations of the standard.