Only 40 years ago scientists tried to understand the post-traumatic stress disorder more and found out, that there was a kind of PTSD which had severe symptoms. This "new" disease had to be classified and couldn't be compared to a "normal" PTSD.
There are many studies and publications, nevertheless there's no uniform diagnosis or code for this disease. Therefore, it's listed as DESNOS - Disorder of Extreme Stress Not Otherwise Specified.