DSM-V: Criteria D
D. The disturbance is not a normal part of a broadly accepted cultural or religious practice. Note: In children, the symptoms are not better explained by imaginary playmates or other fantasy play.
This is a differential diagnosis elimination criteria. This criteria has 2 steps to it:
Cultural or Religious Practice
There are cultures and practices around the world in which (their thought, their subjective reality) a person can become "possessed" by spirits who take over their body and move, dance, deliver messages, etc. Channeling, becoming a loa (channeling a god in voudoun and other practices), angelic possession, etc. would not qualify as DID.
How "Broad" is "Broadly"?
This is an awful wording, because a person's culture can be as large as one tribe, or it can be massive in numbers. To say that the culture must be broad without defining how broad is a flaw of the wording in the book, and extremely unscientific and very subjective.
If the only evidence is routine or typical children's fantasy play, then DID is removed from consideration. Since in DID the experience of being someone else comes from within the person, external "imaginary" circumstances must be supported by other evidence of DID. If you are no longer a child, this would generally not apply to you.
Imaginary playmates are usually experienced as being outside of oneself, and the child does not "act out" their imaginary playmate.
Fantasy play may include things like "I'm a fireman!" and temporarily pretending to be one. However, the fantasy is that the child themself has aged-up and "become" the fireman through whatever rites of passage and training and is now employed themselves as a fireman. This type of play is considered normal and eliminated from consideration if there's no other evidence to support a DID diagnosis.
If a child is diagnosed with DID, then it is likely that child's therapist has witnessed the child having different personality states, memory recall problems, different skills or handwriting (etc.), or the child has disclosed that they themselves have or are a different identity without it being a facet of play.
For example, if the child occasionally asserts that they are another person who happens to be a fireman, talks about the child in 3rd person, has an altered demeanor, etc.
Otherkin & Plural Cultures
Not sure whether these fall under "Broadly" or not, but there are emergent global subcultures in which plurality is not considered inherently disordered, and in which plurals are encouraged to explore their plurality in a positive non-pathologizing framework. These cultures consider plurality only in need of therapeutic assistance if the plural system is in distress and wishes to seek out therapeutic assistance.
This information is not presented as a means of self-diagnosis, but to attempt to explain the criteria to persons already diagnosed who may be confused about how the criteria apply to themselves.