You can add all your customers' data in the customer details. However, it's possible that you use certain unique identifiers (e.g. licence plate number, chassis number) that are not provided by default. Via the submenu ‘Additional fields’ under ‘Customers’ you can add extra fields to your customer details. As soon as it's created you can fill in the field in the customer details.
If you also want to put the additional field on your invoices you first need to add it to the invoice layout. You do this by selecting the submenu ‘Corporate style’ under ‘Settings’ and editing the invoice.
Click in the box where you want to see the extra fields on your invoice and right-click on the extra field under ‘customers’. The additional fields will now be included on your invoice.