By default, the leftmost (when the front of the CEW is facing away from the user) live cartridge is selected. You can use the ARC switch to designate which cartridge you want to use. You do not have to start with the left cartridge.
For example, you can load Smart cartridges of different ranges into the CEW. You might load one 25-foot (7.6-meter) cartridge into the left bay and one 35-foot (10.7-meter) cartridge into the right bay. As shown in the CID below, if the subject is between 25 and 35 feet away, quickly press and release (tap) the ARC switch to select the 35-foot cartridge.
Pressing and releasing the ARC switch quickly selects the second cartridge.