International maritime signal flags - AbsoluteAstronomy.com

The system of international maritime signal flags is one system of flag signals

Flag signals

Flag signals can mean any of various methods of using flags or pennants to send signals. Flags may have individual significance as signals, or two or more flags may be manipulated so that their relative positions convey symbols...

representing individual letters of the alphabet in signal

International Code of Signals

The International Code of Signals is an international system of signals and codes for use by vessels to communicate important messages regarding safety of navigation and related matters. Signals can be sent by flaghoist, signal lamp , flag semaphore, radiotelegraphy, and radiotelephony...

s to or from ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

s. It is a component of the International Code of Signals

International Code of Signals

(INTERCO).
There are various methods by which the 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 vessel
Diving support vessel
A diving support vessel is a ship that is used as a floating base for professional diving projects.Commercial Diving Support Vessels emerged during the 1960s and 1970s when the need arose for diving operations to be performed below and around oil production platforms and associated installations in...

s raise the "A flag" indicating their inability to move from their current location because they have a diver underwater.
one or more flags form a code word whose meaning can be looked up in a code book held by both parties.