For connecting external 12V DC 20-amp max. power supply, such as external battery, or hardwiring to vehicle with charging system. For use with Service-Truck and Bench-Top Alpha MUTT_® models (#5611A, #5611A-RT8, #5610A, #5610A-RT8, #5510A-L4, #5510A, #5510A-L4T10, #5510A-T10, #5510A-L4RT8 and #5510A-RT8).