For forward outrights, settlement on the value date selected in the trade means that even though the trade itself is carried out immediately, there is a small interest rate calculation left. The interest rate differential doesn't usually affect trade considerations unless you plan on holding a position with a large differential for a long period of time. The interest rate differential varies according to the cross you are trading. On the USDCHF, for example, the interest rate differential is quite small, whereas the differential on NOKJPY is large. This is because if you trade e.g. NOKJPY, you get almost 7% (annual) interest in Norway and close to 0% in Japan. So, if you borrow money in Japan, to finance the trade and buying NOK, you have a positive interest rate differential. This differential has to be calculated and added to your account. You can have both a positive and a negative interest rate
