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If the Tax ID is not supplied then tax ID will not be override with NULL.
NewDiscountID: Enter Enter the new Discount ID, or current Discount ID associated with the Item if there is no change. Do not leave blank, if you do all current discount ID's assigned at the item level to the associated item will be removed.If item does not have a discount associated with it leave blank
NewCost: Enter the new cost associated to the item or leave blank if no cost adjustment is necessary. If a new cost is associated to an item this will cause the system to create a receipt as an inventory adjustment on cost of that item.