Dissociation is on a spectrum. At one end we have normal dissociation such as daydreaming or going onto autopilot when doing a mundane or daily task which needs no concentration and at the other we have dissociative identity disorder.

There is peak performance, such as an athlete who is in the zone and then we have dissociative amnesia or fugue which is caused by an extreme psychological trauma (and not by physical trauma, illness or other medical condition). It is a form of dissociative amnesia which is severe and is considered rare.

Then we have Post Traumatic Stress Disorder which can be caused by a traumatic event. There is also a subtype of PTSD which includes derealisation and depersonalisation (these are feelings that the self or surroundings are not real). Complex Post Traumatic Stress Disorder is a more complex form of PTSD which is caused by ongoing stress over a long period.

DDNOS means Dissociative Disorder Not Otherwise Specified, this describes a form of complex dissociative disorder (this is similar to Dissociative Identity Disorder but falls short of meeting the criteria). This can mean that the individual has fragmented parts of the mind which hold traumatic memories and feelings which are too much for the conscious mind to bear, but they are not fully defined personalities as with DID.

https://www.nicabm.com/working-with-structural-dissociation/

DID was formerly Multiple Personality Disorder. In the case of severe abuse at a young age the mind will split multiple times to be able to handle living with ongoing trauma. Each ‘personality’ will hold certain memories and perform a certain role in the system. Internal Family Systems Theory explains this well. Poly-fragmented DID is a person with many alter identities in the dozens or hundreds.

http://traumadissociation.com/index.html#gsc.tab=0

Internal Family Systems Theory and the Structural Dissociation Model explain this: