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: V3 and Legacy 3 - FS44M for channel switching and boost V3 - FS22 for effects loop switching Legacy 3 - FS22 for channels 1-2 and reverb switching V3M and V3MC - FS44L-V3M for channels, reverb, and boost switching. X100B - FS44L for channel switching, reverb, effects loop and boost. VT50, Belair, and Nomad - FS22 for channel switching and reverb.
Bass amplifiers may use the FS22 for mutting the input, graphic EQ switching, and effects loop switching. See models for which features are available.