The LC2i has the ability to split the input signal to so you can send a full range signal to your door speakers, and then send a separate low passed signal to your bass amp.
So, if you purchased the LC2i for powerering a sub amp, you would just use the bass output. But if you end up buying another amp for the door speakers down the road, you could then connect that new amp to the main output of the LC2i.
The bass output has a low pass at about 1 khz, whereas the main output is full range.
The AccuBASS® functionality is only present on the bass output.
Click on the wiring diagrams below to enlarge:
LC2i product page