For example, in USDJPY, the interest rate differential is the difference between short-term U.S. interest rates and short-term Japanese interest rates. If, for example, U.S. interest rates are 5.0% and Japanese interest rates are 1.0%, the interest rate differential is 4.0% (5.0% - 1.0%). This means that if a trader was to sell USDJPY, he would have to pay 4.0% of the notional amount of the contract per year to hold the position. On one lot, the notional amount is $100,000, so the trader would have to pay approximately $4,000 to hold the position for one year. This translates to approximately $11.00 per day per lot for holding the USDJPY position ($4,000/365).
On the GCI system, these amounts are calculated for the trader and shown, as dollars per lot per day, in the "Currency Reference Rates" window under the columns "Prm Buy" and "Prm Sell":
