Depends which you're referring to, and I can only say so much as I don't own much in the way of the new range, but...
The Dark Eldar (and Eldar) language(s) consists of runes, so certain markings you see along the way and in the codex/etc are simply letters/words and the like.
Also each Kabal/Cult/Coven tends to have its own symbol, and I think maybe a lot of the kits have one similar to this emblazoned? Although that's kind of like putting the ultramarines shoulder pad on every marine ever put into production...
A lot, I'm sure, are just decorative.