vent line versus overflow line

I'm in process of putting in an integrated bow sac. I want to tee into the overflow and vent lines of my KGB sac. If I remember right, when my KGB sac overflows, it comes out the Port thru hull. Not sure why though? Is there really any difference between a vent and an overflow? On the KGB sac, the fill/drain line is at the bottom, and the overflow and vent lines come out the top. I don't understand why the overflow only comes out the port when filling? If I turned the bag a little, will it come out the starboard? I'm curious? I'll definately be using check valves in these lines though.