Mine is both. She works with people with PTSD a lot, and is one of the few experts around here on DID. A specialist in any particular disorder may tend to diagnose that disorder more often than other therapists do. It can be because they recognize the symptoms more readily, or because they see what they expect to see.

Interestingly, I don't have DID or PTSD, but my T discussed PTSD-like symptoms when dx came up, and started to do some ego state work with me. She pointed out that PTSD and BPD are also dissociative disorders, and I agree with her on that. I self-diagnose with BPD, and I really believe that it is the dx that is the best fit for me, but she doesn't seem to like BPD as a concept, and apparently doesn't use that one. But I can see how BPD has some features and symptoms in common with PTSD and DID. Mainly dissociation, emotional dysregulation, and trauma history of some kind.

So, specialization can make a difference in how they might conceptualize your symptoms.