Hello @Thej 

This field is determining how a foreign currency invoices should be calculated in case it is paid in different currency. If your invoice is in EUR but your payment currency is USD, when you enter the USD amount system will calculate the amount in EUR based on selected parameter. The same field exist on Mixed Payment window.

Please see help description below.

Recalculation Type:
The recalculation type to be used for the payment. Through this parameter a company can choose a calculation pattern that supports entry of the currency combinations that are currently used. 

When the customer/supplier payment is being matched, if the Amount in Payment Currency field is edited, the recalculation pattern differs depending on the recalculation type. If Recalculate Invoice Amount is selected, the system will recalculate the Amount in Invoice Currency field. If Recalculate Currency Rate is selected, the Currency Rate field will be recalculated.