What is plurality?
Simply put, plurality is the state of having more than one person in a body. The entire group of entities sharing a body is usually called a system.
Plurality is a spectrum. There's no point where someone is definitely plural because it's normal to have parts of self. Parts are linked beliefs, behaviors, memories, and traits. All of a person's parts make up their personality. Parts mean that people that aren't plural can disagree with themselves. They can talk to themselves and act in different ways. They might have an "inner child" or a "work self". Despite all this, they are still one person.
Plurality is more than normal parts of self. A system does not completely fit into being one person. System members can have their own identities, and they may feel like totally different people. They can't control each other. They might hear each other as voices and trade control of their body, and they may even have their own memories. If system members try to ignore each other, huge problems can happen.
Plurality is not always a problem. For some, it can be a very positive aspect of their lives that helps them function better.
What are DID and OSDD?
Some systems have a hard time with daily life because of their plurality. These systems can be diagnosed with DID or OSDD.
DID can be diagnosed when a system of distinct people has serious memory problems (amnesia). Amnesia can mean forgetting large amounts of time, such as not remembering your childhood, but any forgetfulness that is noticeably more than normal counts as amnesia. Forgetting things in the present counts too.
Sometimes, DID is close, but the criteria aren't all met. This is where OSDD-1 comes in. OSDD-1 can be diagnosed if a system has problems with daily life but does not fit a DID diagnosis. OSDD-1 systems might not have amnesia, not trade control of the body, or not be distinct enough for a DID diagnosis. It is a much broader diagnosis than DID because it is less strict.
In some places, Partial DID (P-DID) is a third option. It can be diagnosed if the system meets DID criteria but does not trade control of the body. In those cases, it is diagnosed instead of OSDD-1.