Controls whether a release sample should play if its sustain sample has ended, or not. Allowed values are on and off, default is off - so by default, release samples do not play if their sustain samples have expired.
Although the default behavior makes sense for instrument such as pianos, with guitars there is still some noise when a finger releases a fretted note, so it can make sense to use rt_dead=on there. Also, if using release samples to emulate tremolo playing (so that the note plays when the key is pressed, and again when the key is released - this can be useful for vibraphone, marimba etc.), rt_dead=on can also be useful.