Perhaps it means something other than what I thought it meant?
I think it means that you need to have a power supply capable of supplying the maximum amperage the device needs, without stressing the power supply. A 6A power supply supplying 6A all the time will get hot, and not last as long as you might wish. A 6A power supply supplying 4.2A all the time will be a lot cooler, and last a lot longer.