If you stay within the international transit area and don't pass through Indonesian immigration, you generally don't need to complete the Arrival Card. But if your transit requires you to clear immigration — for example, a terminal change at Jakarta's Soekarno-Hatta Airport, an overnight layover, or a stop where you must collect and re-check your luggage — you're considered to be entering the country and must complete the Arrival Card, just like any other arriving traveler. If you're unsure whether your specific connection requires clearing immigration, check with your airline.