I'll have to see if I still have my manual lying around. It could be that the 6-volt input is for a battery that would keep the DCC decoder functioning when a DCC-equipped Climax is running on analog DC track power. Without that, the decoder (along with the attendant sounds) would shut off if the track voltage dropped below 6 volts.
Again, that's a guess on my part. I noticed that input on my climax when I was installing R/C on it, but never thought to "look in the book" to investigate more.
Bill, I have the Bachmann Forney locomotive and I believe it has a similar plug and play board as the Climax. In the operators manual it states that the 18 volt input is for battery locomotive operation (RC). The 6 volt battery terminal (INPUT BATT 2) is Bachmann quote "If desired, your sound system's + - inputs can also be use to directly power the locomotives chuff circuits using the INPUT BATT 2 screw terminals at the right rear of the board . These terminals have a voltage input range from 5 to 14 volts DC."