That is an A/B switch that acts like a mute.
As it stands, this board gets used mostly with my Friedman Smallbox 50 that has an FX loop, so the signal goes:
Guitar > RMC4 > Fuzz Factory > Klon > A/B Switch > front of Amp
In the FX loop: tuner > GE7 > Memory Man > RV2
The tuner is in the loop...