When you create a new account in Zervant, you will find one example customer (“Customer Company Ltd”) with fictional contact details in your tab >Customer. The VAT number GB123456789 belongs to the fictional customer. If you create a test invoice addressing the example customer, you will see his fake VAT number GB123456789 on the upper right-hand side of the invoice.
Therefore, GB123456789 indicates where your real customer’s VAT number would be shown on the invoice.
Your own VAT number (entered in >My account >Company details) will be displayed in the footnote of every invoice.