It all depends. People can have derealisation / depersonalisation without having any trauma related conditions. Or they can have any of the various sorts of dissociation as part of PTSD. Or have PTSD without much in the way of dissociative symptoms.
Many experts say, and I 'm inclined to agree, that PTSD is by nature a dissociative condition, that we develop because we store our trauma separately from out usual experience - we diss - associate it from ourselves - and then it comes bursting out at inconvenient moments. But I think other regular posters would disagree with that view.
I think what it comes down to is that we have very different experiences of the condition. Why do you ask?