ISA (Foreign Conversion) Fees have two different calculations based on whether the transaction being processed is being done in US funds ('no conv' as displayed on BIN configuration screen) or will require a conversion from the 'foreign' country's currency to US funds (w/conv as displayed on BIN configuration screen) for processing.

The way it is determined which fee should be calculated is within the 'transaction' message that your vendor sends to CU*BASE for processing.  Within the message the vendor sends to CU*BASE there is an indicator (with VISA it is  a 'C' or an 'S') and based on that indicator, CU*BASE performs the correct calculation and processes it against the member's account for that transaction.