Simplify the following text for a Grade 9 native English-speaking person.
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: In air forces the serial number is used to identify a specific aircraft . 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 ) .