It is legally required to display the Client Tax Number on invoices in some countries. You can add this information to the bottom of your client’s address to have it show up on the invoice.
To add this, go to Contacts > External Contacts and search for your Client’s Company name. Add the Tax Number under their address in the “Override Display Address” field.
If you’like to add it above the address, you can also use the structured address fields and add the tax number to the “Address Line” field.
Click Save. When you next create an invoice for this customer, the Client Tax Number will be displayed under their address.