Overview

firstly, what is DID? DID stands for Dissociative Identity Disorder, and it is a very complex trauma disorder where you go through such an extreme traumatic experience at a young age, when your personality is first forming (younger than age 7), that your mind goes through such an extreme trauma (imagine your brain saying "nope, i'm not going to live, i'm going to die") that it can not handle it and instead of breaking completely, or having another disorder (typically a trauma disorder), you split off, and create what are called Alters

to start off, let's go over some terms and their related information

Alters are the terms for the different (people/personalities/facets) of the system, this is the proper term and the one in parentheses are simply common terms that someone might use if they don't understand. Alters typically have different functions in a system, called System Roles (more info in System Dynamics), for example, Self Helpers may act as a "therapist" for the system, talking to Alters, or even going so far as to have sessions and give something relatively similar to therapy, however, this is not therapy done by a professional unless that Alter has gotten a degree in real life, and not in the Inner World, seek professional help if you wish to have therapy, however sessions with an internal helper may be useful when switching therapists, as there are many similarities. Protectors will protect the system from toxic people, dangerous situations, etc by either taking/dealing with abuse or trauma, or entirely fighting back against it, this depends on the situation and Alter. Trauma Holders may hold the memories of trauma that the system went through, so as to keep the system from feeling too much pain, to sort of "Compartmentalize" the trauma so it doesn't hurt the system as a whole, among other roles.

a System is the term for a collective of Alters, for example, us Alters are Nebula System, which is our collective name, or, System Name, these are really cool or interesting names, and tend to be or relate to things that have many variations, like for us, there's gases and colors, and all sorts of other "things" in nebulas, like there's many different Alters in our system of different types, genders, etc. other examples may be Celestial System, who, and this is a direct quote, "found certain aspects in that name to fit who we are as alters and with what our names mean", another example is The Punk Rock System, who chose that name because "the majority in our system LOVE rock music and we felt that punk rock system would be a great name for our system"

another term is Groundwork, which means self discovery that an Alter will do to discover themselves and find out who they are, and build their sense of self, and really solidify who they are, both within the system, and to others, and to themselves.

these are just a few simple terms.

one other term you may hear is "Singlet" which is used to refer to someone who is not a system, someone without Alters, who is singular, like most people reading this will be.

some additional notes:
1: mental age of an Alter is NOT always the same as the age of the body that their system resides in (the age that would be calculated using the date on their birth certificate), so if that alter gives you one age, ask for clarification on the body's age and the Alter's mental age if it is needed (such as for consent in NSFW/interpersonal interactions, or any other situation where the age of a person or Alter is needed)

2: Alters are separate, and should be kept that way unless stated otherwise, meaning don't tell one Alter about doing stuff with another Alter that they do not wish to know or talk about, and some Alters may wish to speak about different subjects and be ok with some and less ok with others, and that can change from Alter to Alter. they're part of the system, but also their own individual, respect their boundaries, and the boundaries of the system, and try to be aware of which Alter you are speaking to if they tell you

3: in-system abuse can be a real thing, and some Alters may be a huge problem, or set up rules that are unjust, and if you're a system, keep an eye for these behaviours, and remember: having a healthy, or at least good and non-abusive system dynamic is essential in the long run, i know it can take years, but it's necessary for you and your system.