3 Answers

http://answers.yahoo.com/question/index?qid=20061030180440AAeibLI

Not sure of what you mean but I am thinking you mean 'line out and line in??  if so then the line out is for aux speakers, usually if memory serves me 7.75 volt.  The line-in is for auxiliary input device like iphone, cellphones, other audio equipment that will use your audio amps amplification and speakers.

This is a simple answer that when you think about it will make sense. Lets say you purchace  another amplifier for your mids&highs.But the shop that sold u the head unit neglected to tell you that the radio only had 1 pr. of rca's , The rca outs on the amplifire takes care of this situition.Just plug tthe rcas into the out on the amplifier and connect them to your new amplifier. Done, no buyin another radio,no xovers, no nothen except the proper gauge powe and ground wire.And jus like that your off witth your windows down and your new amp SINGING.

  I HOPED THIS CLEARED IT UP A SMALL BIT. THANK YOU                     R.ATKINSON