About Tax Codes
The tax codes on the header (CO/PO) are the codes used to calculate taxes where the line item tax code is blank. However, the tax codes on the header are not defaulted to the line items. The codes on the line item are ALWAYS defaulted from the Items form.
For tax systems with Tax Mode set to Item, the defaulting begins at the Tax System form. When a new item is entered on the Items form, the item tax code gets set to the tax code in the default item tax code.
From this point, any time a new line item is added, whether in area or item mode, the new items' tax code(s) get set to the tax code(s) in Items, except in the case where a special (for example, non-zero tax rate) exempt code is entered on the header form.