Many of Carvin's guitar and bass amplifiers make use of external footswitches for switching channels, effects and reverb on or off, and Boost. For more details check the amplifier you are purchasing.
Here is a short list of heads and amplifiers and their footswitch use:
FS44M used with: V3(With internal reverb) and VL300 (Legacy 3) FS44L-V3M used with: V3M and V3MC FS44L used with: X100B (Note: only for current X100B series IV. Not compatible with the 1980's and 1990's X100B versions.) FS22 used with: VT50, Belair, Nomad, and several bass amplifiers and mixers.
Bass amplifiers may use the FS22 for mutting the input, graphic EQ switching, and effects loop switching. See models for which features are available.