In the menu *Invoicing > Configuration > Settings*, check these options:

* *XML Format embedded in PDF invoice*: if you want to generate Factur-X invoices, this option must be set to *Factur-X (CII)*
* the *Factur-X Level*: unless you have a good reason, you should keep the default value *EN 16931 (Comfort)*
* *Factur-X Refund Type*: choose the type of the XML invoice for customer refunds (keep the default value if you are not familiar with this setting).