You can look at this Depersonalization - Wikipedia for more info, but I have experienced depersonalization as my "car knowing where to go," that is, I get to work, but have no recollection of driving there. Sometimes I look at my hand and it seems like it doesn't belong to me. I have had the experience of "day-dreaming" in my t session, and my T asks me where I went; once I was on the ceiling, looking down at my session. Dissociation, on the other hand, has to be "At least two distinct and relatively enduring personality states," whether they are called alters, others, or another name. I have never experienced this, nor has my T mentioned that "anyone else" has "come out" during my sessions.