(covers information from several alternate timelines)
A Starfleet ID code was an identification used by Starfleet officers. The code was classified to civilians and known only to members of Starfleet.
In an alternate timeline in 2374, a captured Tom Paris sent a message along with a Starfleet ID code to the severely damaged USS Voyager from Annorax's weapon ship. Captain Kathryn Janeway was convinced the code could only have come from Paris since the ID code was classified. The message with the code contained the coordinates of the weapon ship, which Janeway later destroyed, with help from the Nihydron and the Mawasi. (VOY: "Year of Hell, Part II")