Your guess is correct. It is in the manual, but the parameter is listed as “Loop Decay Rate”. I’ll change that to “Decay” in the next version of the manual to avoid confusion.
Loop Decay Rate: When dubbing you may want the original saved audio to persist as you add new sounds. Of course, indefinitely adding new signals will eventually result in ‘mud’ (the “Crayola” effect). The Decay Rate control allows the saved audio to fade as you dub new material. The Decay Rate is adjustable from 0% [DCY: 0] to 100% [DCY:100]. When set to 0%, the loop never decays. When set to 100% the previously saved audio decays completely each time through the loop when dubbing. In other words, the looped audio is only played once. The Loop Decay Rate control has no effect on normal Playback, only dubbing.