Its just "free channel" marker. Some other coastal maritime stations send them. But sometimes on this freq station send weather information in mode SITOR-B and provide telex service for ships in mode SITOR-A. You can receive and decode ship to coastal station communication. But on modern ships hf antennas not so good and hf transmitter power only 250 watts, so i can decode only coastal stations answers for ship station in SITOR-A