What is the difference between the servo track and the line track of a motorized potentiometer?
I would suppose that pins 123 are what you use for the audio or other analog channel you're controlling, and 1'2'3' are what the motor/system uses to figure out where it is on the slide. A similar device on the Sparkfun site explains:

"The slide contains two separate 10k linear taper potentiometers so that you can use one as servo-feedback in order to read the position of the slider and use the other to control whatever your target is"