If you have selected LB/FB/LBI/FBI as Payment method (Pmt. method) you should here enter the amount to pay. This amount is entered in the currency used for the supplier in question, but can be changed if needed.
The currency is selected in the field to the right. If you enter an amount in a foreign currency, a field appears where you can enter the conversion factor (exchange rate), which was valid when the payment was made. The amount that has been entered in a foreign currency, will be shown converted to the company currency below this field.
You register and update currencies in the Currencies procedure in the Global Settings.