The history of the use of the Jolly Roger flag on submarines is here: https://en.wikipedia.org/wiki/Use_of_the_Jolly_Roger_by_submarines  It’s meaning when used on submarines is the same worldwide — “victory” or “mission accomplished”. The meaning is discussed in the first paragraph. Use by other navies ...”The practice, while commonly associated with British submarines, is not restricted to them. During World War II, Allied submariners working with Royal Navy fleets adopted the process from their British counterparts.[12] While operating in the Mediterranean, the Polish submarines ORP Sokół and ORP Dzik were presented with Jolly Rogers by General Władysław Sikorski, and continued to update them during the war.[13][14] At least one British surface ship recorded their U-boat kills through silhouettes on a Jolly Roger.[15] The Royal Australian Navy has also flown the Jolly Roger from submarines on occasion. Following the first Australian live firing of a Mark 48 torpedo in 1987, HMAS Ovens used the flag to indicate the successful sinking of the target ship Colac.[16][17] HMAS Onslow flew the Jolly Roger in 1980, following her successful participation in the Kangaroo 3 wargame as an opposing submarine: the flag bore the silhouettes of the seven surface ships involved, as during the exercise, Onslow had successfully 'sunk' all seven.[18] At least twice in 2017, the USS Jimmy Carter, a U.S. Navy attack submarine which has been modified to support special forces operations, has returned to its home port flying a Jolly Roger.[19] The reason for the flag in both cases is not publicly known....”