This article is from the Dissociation FAQ, by Discord (firstname.lastname@example.org) with numerous contributions by others.
DDNOS is a convenient diagnostic label used to mean that the disorder,
while not matching any other disorder, involves dissociation. People
with partial symptoms of the above disorders might be diagnosed as DDNOS.
Because this is a purely diagnostic category, there is no way to actually
define it; you might, however, see or hear people mention that this is
how their therapist has diagnosed them. A common use of this category
is when a person does not meet the diagnostic criteria of MPD, but
exhibits most of the symptoms and history of someone with MPD.