Currency exchange orders are used for sending orders to Bricknode Financial Systems where one currency (Cash asset) is sold and another currency is bought. You can decide if you want to enter how much of a currency you want to buy or how much of a currency you want to sell. These are called the two legs of the order.
If you define how much you want to buy of a currency the amount that is eventually sold of the other currency is decided depending on the resulting price (exchange rate) that the order is executed with.