Flag Signals:
There are various methods by which flags can be used as signals:
Each flag spells an alphabetic message, letter by letter.
Individual flags have specific and standard meanings; for example, diving support vessels raise the "A" flag indicating their inability to move from their current location because they have a diver underwater and to warn other vessels to keep clear.
One or more flags form a code word whose meaning can be looked up in a code book held by both parties. An example is the Popham numeric code used at the Battle of Trafalgar.
In yacht racing and dinghy racing, flags have other meanings; for example, the P flag is used as the "preparatory" flag to indicate an imminent start, and the S flag means "shortened course".
