Le signal distinctif est le groupe de lettres et de chiffres assigné à chaque station par son administration. Par station, on entend un navire, un aéronef, ...
www.shom.fr/fr_page/fr_prod.../32_1.002_19032010.pdf

Merchant vessels are assigned call signs by their national licensing authorities... call signs for larger vessels consist of the national prefix plus three letters (for example, 3LXY, and sometimes followed by a number, i.e. 3Lxy2). United States civilian vessels are given call signs beginning with the letters "W" or "K". Originally both ships and broadcast stations were given call signs in this series consisting of three or four letters, but as demand for both marine radio and broadcast call signs grew, gradually American-flagged vessels were given longer call signs with mixed letters and numbers.

http://en.wikipedia.org/wiki/Call_sign#Ships_and_boats