It is actually not a pocket. I put a sturdy material there (VX21 in this case, but I have used DX40 in the past) so that when things are strapped in between the daisy chains, there will be greater abrasion resistance. This way I can use a lighter material on the majority of the pack, while still having a tough area for lashing gear. You can use shock cord or velcro or straps (switch the compression from the side to the front) to secure things.
I do not have a picture of this stay system. I have a picture in my flickr account of a different one though. They are slightly different though. I used vx21 for the stay sleeves on the green pack. I also used a wider velcro, since this one has load lifters. I wanted more holding power. The title of the album is TX07 pack.