What I know is that DES-NOS is the most "severe" form of PTSD. (not implying that there is now a competition going on in who has the worst diagnosis).
This will probably be a new diagnosis in the DSM V.
They've decided to do this for the people who have almost every symptom in some degree mentioned in the DSM IV and can therefore be classified with almost every psychiatric disease you can think of.
(Hence the many misdiagnosis of severe ptsd people.)
Because this was "ridiculous" they found out that there must be something else going on with this group, therefore calling it DES-NOS or complex PTSD which is the same.
We shall see when the DSM V is available how they made their classifications. It implicates even more the ignorance of psychiatrists what to do with this scale of symptoms and how to treat these people.