The Line Item Detail - Tax Code field displays the Tax Code from the item's Inventory database record.
To edit the Tax Code, press [Ins] from the Tax Code field on the Line Item Detail screen and select the desired Tax Code from the Tax Codes Lookup List. Editing the Tax Code on the Line Item Detail screen changes the Tax Code for the current line item only. It does not change the Tax Code field in the item's Inventory database record.
See:
Line Item Detail Screen Contents
Sales Manager Contents