What is the reason for using the term "sailing" instead of "flying" when referring to a ship under the flag of another country..

The term "sailing" is used when referring to a ship under the flag of another country because it emphasizes the legal concept known as "flag state jurisdiction" or "flag state control."

When a ship is registered under the flag of a particular country, it is subject to the laws and regulations of that country, regardless of where it is operating in the world. This concept is similar to how a person must abide by the laws of their own country even when traveling abroad.

So, when you say a ship is "sailing" under the flag of another country, it means that the ship is registered with that country and is subject to its laws and regulations while it is at sea.

On the other hand, the term "flying" is often used in the context of aircraft, where the national flag of the aircraft's registration is displayed on the tail or fuselage. In this case, the term "flying" is used because aircraft operate in the air rather than on the water.