account-financial-tools/account_fiscal_position_vat_check/readme/DESCRIPTION.rst

With this module, when a user tries to validate a customer invoice or refund
with a fiscal position that requires VAT, Odoo blocks the validation of the invoice
if the customer doesn't have a VAT number in Odoo.

.. figure:: static/description/vat_check_invoice_validation.png
   :alt: Error upon customer invoice validation

In the European Union (EU), when an EU company sends an invoice to
another EU company in another country, it can invoice without VAT
(most of the time) but the VAT number of the customer must be displayed
on the invoice.