Today's flag is: Quebec
As a letter, that is the letter Q. As a message, it means:
My vessel is “healthy” and I request free pratique.
[Merriam-Webster pratique definition: clearance given an incoming ship by the health authority of a port]
boof (OP) double-posted this 1 year ago, 1 day later, 3 days after the original post[^][v]#1,330,393
The declassified NATO information for flag Quebec:
indication: BOAT RECALL
normally displayed: Where best seen.
WHILE FLYING: meaning: All boats belonging to this ship or boat(s) addressed return to this ship immediately.
For use in submarine exercises, see Article 1306b.
[Article 1306b - Flag QUEBEC/Submarine/Disregard me. I am not open to attack. I am not to be reported.]
boof (OP) triple-posted this 1 year ago, 1 day later, 4 days after the original post[^][v]#1,330,484
There is some similarity with the Quebec flag and the caution flag used in motor racing. Also, the municipalities of Basud and Paracale (in Camarines Norte, Bicol region, Island of Luzon, Philippines) have yellow flags.