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Yellow flag other meanings from Wiki-

The Yellow Jack ("Quebec") signal flag, is a plain yellow banner that was historically used to signify a vessel was, or might be, harboring a dangerous disease and needed to be quarantined (the flag represents the letter “Q”). Later the flag came to mean the opposite, signaling: "My vessel is healthy and I request free pratique".

A "Double Quebec", flying from a ship would signal" "I require health clearance"
A ship flying two Quebecs ("QQ"), or "Double Quebec", is signaling: "I require health clearance". In both cases, if and when free pratique is granted, the vessel may lower the Quebec(s), raise the national ensign of the port, and do business there. In the event that, for health reasons, the vessel is not granted free pratique, it continues to fly the Quebec, in effect indicating that it is in quarantine until such time as any health concern is resolved.

In international maritime signal flags, plain yellow, green, and black flags have been used to symbolize disease in ships and ports. The color yellow has a longer history. It was used to mark houses of infection prior to its maritime use.

Yellow jack also became a name for yellow fever. Cholera-infected ships also used a yellow flag.[1]

The plain yellow flag ("Q" or "Quebec" in international maritime signal flags), may derive its letter symbol from its initial use in "quarantine"[2]