A buoy carrying a copy of a starship's logs, launched when the ship is in danger of destruction so that Starfleet may learn what happened.
A recorder marker is a small buoy holding a ship's logs and sensor records. A captain ejects it when the vessel may be lost, in the hope that another ship will one day pick it up.
One starts the story of Where No Man Has Gone Before. The Enterprise recovers the old-style marker of the SS Valiant, lost two centuries earlier at the edge of the galaxy, and its damaged tapes tell of a crew member who gained strange powers and of a captain who finally ordered his own ship destroyed. The warning does not stop Kirk from attempting the barrier himself.
In The Corbomite Maneuver, with the Enterprise held by Balok's vast Fesarius and given minutes to live, Kirk launches a recorder marker. Balok destroys it at once, to show that not even a record of the encounter will be allowed to escape.