The VAT number needs to match a valid format, which may appear differently for each country. The European Commission provides an overview for these formats. Billomat only stores data that correspond to this system.
Alone the fact that the number fits into this format does not confirm its existence. A check can be performed via the link next to the field, after a customer has provided a VAT number. The link is matched by a live interface to the databases of the member countries.
Hint: If your own company has a VAT number assigned, you will receive an additional unique identification number to use for further documentation with the tax office.