A fin flash is part of the national markings

Military aircraft insignia

Military aircraft insignia are insignia applied to military aircraft to identify the nation or branch of military service to which the aircraft belongs...

 of the military aircraft of a number of countries.

In addition to the insignia displayed on the wings and fuselage, usually in the form of roundels, an additional marking known as a fin flash may also be displayed on the fin

Vertical stabilizer

The vertical stabilizers, vertical stabilisers, or fins, of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip. It is analogical to a skeg on boats and ships.On aircraft, vertical stabilizers generally point upwards...

. A fin flash often takes the form of vertical or slanted stripes in the same colours as the main insignia

Military aircraft insignia

Military aircraft insignia are insignia applied to military aircraft to identify the nation or branch of military service to which the aircraft belongs...

. Alternatively, a national flag may be used on the fin.