@ragtop -- it probably does not matter. Try port #1. If it works, it works.
Do you have a "Home Security Network" ? If not, then you can safely ignore this feature of the cable-modem. My automobile has both "front" and "back" speakers, but I've never needed to access the control to change the front/back volume mix, since it seems fine "as is". My ignorance is bliss. ๐