If this option is selected, the sales tax amount that you enter or that is sent with the vendor invoice through a web service ...

If this option is selected, the sales tax amount that you enter or that is sent with the vendor invoice through a web service is used instead of the calculated sales tax amount based on the purchase order. Clear this option to automatically recalculate taxes if changes are made to the invoice. If this option is selected and no overrides are entered, the sales tax amount depends on the selection in the Sales tax direction field in the Sales tax transactions page. If the Sales tax receivable option is selected, and if the Ignore calculated sales tax option is selected, the sales tax amount is 0.00. If the Sales tax receivable option is not selected, or if the Ignore calculated sales tax option is cleared, the calculated sales tax amount is used. This field is available only if the Keep sales tax adjustments for PO invoices option is selected in the Accounts payable parameters page.