Original: In air forces the serial number is used to uniquely identify individual aircraft and is usually painted on both sides of the aircraft fuselage, most often in the tail area, although in some cases the serial is painted on the side of the aircraft's fin/rudder(s).
Simplified: It is usually painted on both sides of the aircraft, most often in the tail area, although in some cases the serial is painted on the side of the aircraft's fin/rudder(s).