DSM-V: Criteria C
C. The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.
This puts the 2nd D in DID. Without impairment, it's not a disorder; it's just a difference.
Starting at the easy end of the criteria: "in social, occupational or other important areas of functioning" means that you can take care of your basic needs without problems — applying this to school, work, home, training programs, significant relationships in your life (like maintaining an appropriate support system), skills of daily living (eating, bathing, getting dressed, grooming, bodily functions), emotional and physical needs, and personal gratification such as having and achieving goals, feeling you have a significant place in the world, feeling fulfilled, and being contented.
So what is clinically significant distress or impairment?
Distress would be anxiety, "bad" stress (as opposed to eustress or "good" stress such as excitement about going on vacation), guilt, sorrow, depression, pain or heartache, unease, etc. Clinically significant means it's not just something that happened one day and went away. It means it's significant enough that the distress itself may be causing more distress. It's significant enough that if someone told you they feel this way, you might suggest they see a therapist. One ordinarily does not subject oneself to therapy unless you're distressed, so this is almost a slam-dunk criteria the moment one appears in the therapist's office if it is a voluntary appointment.
Impairment in the case of DID would be forgetting appointments or missing work because you don't realize what day it is, explosive outbursts that affect home or business life, gaps in memory of significant events that ordinarily would be memorable (like a meeting with an important client, your wedding day, getting a promotion, etc.), insomnia, etc. It could be significant if one became a little who doesn't know how to bathe or cook or feed themselves properly. The possibilities are endless, but it must have a negative impact on one's life and independent living skills in some way. In today's parlance, it could also be significant issues with "adulting".
Putting it Together
When you add up impairment or distress + all the significant areas of life, this criteria is basically saying "do you have your stuff together or not?" Are you adulting competently? Do you have repeated trouble in areas of your life that cause you significant pain or distress?
Unfortunately this area as a criteria for DID bumps heads with a significant diagnostic label for another set of criteria: complex post-traumatic stress disorder (C-PTSD). There is a significant overlap between DID and C-PTSD, and C-PTSD is not currently included in the DSM V but is up for inclusion in the next version of the International Classification of Diseases (ICD, the international diagnostic standards by the World Health Organization/WHO). One day we may have a different label for a mental difference of having many identities/people in one's head versus the label for chronic abuse survivors (C-PTSD) and a subset of clinical considerations for when the 2 overlap.
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.