Interest is the principal times the rate times the number of days in the period divided by the number of days in a year. Consider a hypothetical example in which the principal is 100 million yen, the rate is 2%, and the period is 169 actual days. Act/360 divides 169 by 360 and gives interest of 938,889 yen. Act/365F divides by 365 and gives 926,027 yen.
Japan Securities Clearing Corporation limits the day count for the floating rate side of the yen interest rate swaps it clears to "Actual/365(Fixed)", and lists this condition under the item "日数計算式" (day count fraction).