Field help (KAMTVATCUR) 

This field displays the invoiced VAT amount, expressed in the transaction currency. It is automatically calculated based on the ex-tax amount and invoiced VAT rate. The amount then calculated can be modified by the user (except if the expense comes from the entry of a MO-type document). The entry of a VAT amount equal to 0 is allowed. In that case, the rate is automatically set to 0.
If the expense originates from the validation of an invoice or validation of an accounting document, this value is not calculated from the displayed rate: it is forced with the invoiced VAT amount of the source document.