line coupler hierarchy

I'm fairly new to ETS.
been trying to add line couplers to my system, and I have encountered some problems.

- when I add a line coupler, it does one of two things:
 1. when I add the coupler to the backbone, to couple the backbone to the area, it automatically creates a backbone segment and places the coupler in there. what does this segment mean? 
 2. when I add the coupler to the area, to couple the main line to the line, it first lets me place 1 coupler in the area, and then any other coupler is again placed in a segment
   for example, in this topology:
   

-how does the line coupler know which line it couples? is it something I need to configure? if so, where do I do it?
is it something in the topology structure? if so, how do I make sure it is built correctly?

my questions are regarding area couplers (backbone to area), line couplers (area to line), and line repeaters (line to seg.)

thank you very much!!