When you create a new customer, the standard VAT rate in the customer information sheet is set to the standard rate in your country (21% in Belgium). If you want to set a different standard VAT rate for your new customer (e.g. because you mainly work with renovations at 6%), you must first adjust the standard rate in your own company profile.
To do this, click on 'Profile', at the top of the side menu, and adjust the VAT rate in the 'VAT' box. Do not forget to click 'Save' afterwards.
When you now create a new customer, the VAT percentage for that customer will automatically be set to the chosen percentage.