I thought perhaps it was an End-of-Transmission beep ("Courtesy Beep"), but it doesn't seem to do that even when END.ON. There's no beep.
But careful listening (on another receiver) reveals that this selection (END.ON) does slightly extend the transmission time by about 0.5 seconds past the release of the PTT button (as compared to END.OFF).
So everything is there for a Courtesy Beep except the beep itself.
A totally-silent courtesy beep... Nice.