Games Workshop have released what looks to me to be an experiment in Codex design; release a generic Codex that covers all bases and then release Legion/Craftworld/whatever supplements that contain specific and fluff-based rules to really theme a force and their acid test is the Iyanden supplement. From what I understand it basically contains a whole bunch of rules to make an Iyanden Eldar army perform much more like they'd be expected to based on their back story and (again I'm not 100% sure but this is what I understand) they alter the Force Organisation Chart or alter how the units fit into it or something like that.
