A VAT number is a registered tax identification number in tax systems that use Value-Added Tax (VAT). If you're VAT registered, you must issue VAT invoices. Including VAT on your invoice tells a client how much VAT they paid for a service. This is important information because some clients may be able to claim that VAT back.
To add your VAT number on Bookem, go to Settings > Payments and Invoicing
Add your VAT number under Additional Business Information
Invoicing is a Bookem Plus and Premium feature, read more here