each country has their own definitions of what is and isnt a mental disorder. here in my location (USA) the difference is that DID is a mental disorder where you have two or more special types of alternate personalities. depersonalization/derealization does not include having to have two or more alternate personalities. short version depersonalization /derealization is things like feeling numb, spaced out, disconnected, like an observer rather than a participant. with depersonalization these feelings are perceived as being inside your self ( I feel numb, I feel spaced out, i feel like I am moving out of sinc with everyone /everything) where as derealization is feeling those outside yourself is unreal dreamlike, moving at a pace different than the way you normally are. (everyone else seems to be talking too fast, moving too slow or fast....the world feels unreal rather than I feel unreal...)