Jumper mode 303.

Citation : In 303 mode:
It controls how the final amplitude of the VCA signal fades away when notes are set to sustain for longer periods of time or multiple notes have been set to glide to one another. So the Revolution acts like the original 303.

On the original 303 if you tried to sustain for a longer period of time, the note amplitude would eventually fade to zero.

In Revolution mode:
It allows the notes to remain sounding for as long as a sustain note is written, or if multiple notes are written to glide to one another for long periods of time, it will allow for infinite droning tones.