C.M = cargo manager
E.D = external distributor
ADRs = advanced detector rail.
I wanted to have a SC2 tree farm that handled 2 sapling types (oak and force). I laid out a track that had 2 main loops that were connected by a single line of rail, in addition to a small utility loop (where I could hold the cart and disable it and remove everything from the inventory). Each main loop had its own C.M with an E.D that was placed on the inside of a corner on the loop and acted on 2 ADRs.
The EDs are basically just a proxy for the CM (think of the transvector interface) and it really comes into play when you split up the CM's inventory into quarters using one of the buttons in the CM gui (I think it is the middle one). The colours on the side of the ED gui represent the sides of ED and the icons at the bottom refer to a part of the CMs inventory. I don't remember the exact labels because I don't have minecraft open right now but the labels include stuff like top-right, bottom-left etc. You can also make it look at the colour coding which can be useful. When you drag and drop one of these icons a particular side of the the ED, designated by the colour code, you establish sided access to a specific part of the CMs inventory which can be accessed with your preferred piping solution (I really like logistics pipes for large Steve's Carts set ups by the way, my tree farm used it.).
So, back to my tree farm example. I wanted to keep the saplings that were available to the woodcutter separate depending on which loop he was in, so if he was in the oak section he could never plant force trees and vice versa. Whenever a cart left a loop it would drop off all its inventory and all saplings in the cutter (and possibly fuel as well) which would then be piped out of one side of the ED. Then there was a passive supplier chassis on another side of the ED which would stock 64 saplings on the loop entry side of the CM so that when the cart returned to this loop it would get its saplings back (and any excess went to the default route by the way). You dont have to sink 2 stacks of saplings into the cutter this way either which is nice. I also had some piping that went into one of the sides of the CM (via ED so it went into the right corner of the CM) that would keep about a stack of bones supplied for the fertiliser module.
I don't have screenshots right now but if you ask for some I might get some to help explain it better.