This is similar to the question of which country's passport a tourist may be carrying. Compared to the countless indecipherable emblems that airplanes carry, the national flag aboard a shipping vessel is an "irrefutable proof of nationality" that is hard to mistake.
所謂「旗」，是指輪船掛的國旗，好像旅客持用某一國的護照一樣；比起飛機機身上千奇百怪的各式圖案，輪船上的國旗更是「國籍確鑿」、難以蒙混。