Signal flags are "signs that float" in the wind used to communicate between ships or with the land, according to the rules of the International Code of Signals.
The Code includes the twenty-six letters of the alphabet, the ten numbers from zero to nine, an "answering" signal and two "substitutes". For certain maritime festivals, all the Code flags are hoisted as decoration running fore and aft and to the mast tops.
From left to right: Alpha - Bravo - Charlie - Delta - Foxtrot
For racing, certain signal flags are used for starting procedures or warnings, on land and at sea. The "number" flags can indicate the leg number of the regatta. The "answering" signal interrupts a starting procedure or delay.
The "P" flag is the "preparation" signal before a start. The "X" flag means that one or more yachts crossed the starting line too soon and must go back to recross it. The "1st substitute" means a general recall: too many boats jumped the gun and a new procedure will be started.