Click Inventory management> Item details .

On the Overviewtab, select an item.

On the Referencestab, in the Purchase ordersection, select a value in the Item sales tax grouplookup field. This value is the default item sales tax group for the selected item when it is entered on a line in a purchase order.

In the Sales ordersection, select a value in the Item sales tax grouplookup field. This value is the default item sales tax group for the selected item when it is entered on a line in a sales order.