You can define the basis of tax calculation in IFS/Sales for a given customer, and in IFS/Purchase for a given supplier. The basis defined for the customer or supplier is used as the default value when creating customer orders or purchase orders for that particular customer or supplier.
The Use Price Incl Tax option in the Customer and Supplier pages determines the tax calculation basis for each company that you list. The default value of this option is fetched from the company added. It is possible to change this value if required. When a new customer or supplier is created using a template customer or supplier, the value of this option is inherited to the new customer or supplier from the template.
When creating a new customer order or purchase order, the default tax calculation basis value is fetched from the particular customer or supplier.