Community Terminology
Plurality as an experience is very diverse, and as such there are sub-communities within the overall big umbrella of the plural community. It's helpful to look at the big umbrella then to look at the sub-communities and divides within the big umbrella.
Identity vs. Experience
One thing to be clear about is that there's some measure of arguments about the difference between identity and simple experience descriptors.
Identity says "this is an intrinsic and inseparable part of me and thus I accept it and learn to live with it." Identity comes with an implication of culture, potentially comes with pride (as opposed to shame), and at least a clear goal of self-acceptance even if it has not been fully realized.
Experience descriptors are words used to help describe experiences as in "this is something I experience, and I may or may not accept it as an intrinsic and unalterable part of my being." Descriptors do not necessarily come with an implication of pride or shame; they are neutral.
Someone may be autistic (descriptor, neutral) or Autistic (identity, pride — shown here capitalized for clarity only). Capitalization of a pride marker here is following the convention of the deaf (sensory disability) vs Deaf (culturally Deaf, pride marker) community for purposes of clarity as to which definition is being used on this site.
The Plural Umbrella
People who experience "being many" in any way are plural. Subjective and reported experiences (encompassing what one might call "plural experiences") across the umbrella are generally similar, with a variety of soft and hard divisions within the community based on whether folk have certain beliefs or not.
One set of divisive beliefs is how plurality may form. Much like parallels in the professional community (whether DID exists, is fake, is iatrogenic and imposed in clients, is delusional, false memories exist, etc.), there's divides based on both subjective belief (of how one's own plurality came about) and on the mechanisms with which plurality overall can develop for people, including whether one can become plural on purpose, be born plural, whether one can be plural without trauma, etc.
Nearly everyone across all of plurality asserts that each individual plural system is unique, yet when folk say "Does anyone else (have this experience)…?" invariably there's a chorus of answers in any given subset of the community, although in some communities the experience itself may be a subject of gatekeeping and the questioner ousted from that insular community.
The Inclusive Plural Community
There's a number of plural folk, across the whole spectrum of subjective experiences and beliefs regarding their own development, who consider themselves to be "inclusive plurals" and assert that there's a larger umbrella around plural experiences and that there's not a lot of difference in the variety of experiences of being plural that prevent mutual support, inclusion, communication, comparing experiences, sharing materials and learning from each other. These folk are Plurals or perhaps remain in the larger inclusive Plural community even if they do not primarily use the term as their identity marker.
"Multiple" as an Identity Term
An older term for people with plural experiences is "multiple" and there are folk with & without DID or OSDD who continue to use the term. Some consider "multiple" to be a medically-derived term and don't like it (they prefer a term independent of diagnosis implications and in control of the community not professionals). Others assert that multiple is a community term even if it was coopted from the medical community, and do not wish to change their identity marker. Many insist that "multiple" is also for inclusive plurality, and are generally older systems used to the term, and reluctant to pick either System (the newest term) or Plural (which may have been around almost as long as "multiple") as an identity marker.
Many resources on this site still use "multiple" as implied in the title of this section of the website ("Multiplicity: The Missing Manual").
"Plural" as an Identity Term
Plural was coined long ago, its origins for now buried or lost in Usenet or offline resources in the 1980s or early 1990s. In the mid-to-late 1990s otherkin multiples picked up the term, which became popular via Dark Personalities forums and slowly spread out to other sectors of the community over time. There was a huge uptake of the term following an event on Twitch in 2019 that became the Twitter hashtag #pluralgang which remains an inclusive community callsign on Twitter alongside #PluralPride and others. The community has grown a great deal since then with more visible and explicit presence on nearly every platform.
People who identify with plurality (as a culture) may use the Plural (capitalized here as in Plural Pride) identity marker. Folk in the wider umbrella of experiences of "being many" may occasionally say they are "plurals" (uncapitalized here, not necessarily an identity marker but a simple descriptor).
Plurals, on the other hand, include a much broader range of experiences, with or without dysfunction or trauma. Consider parallels to the Hearing Voices movement, where folk do not medicalize their experiences. Plurality does not require trauma. Arguments have been made within the professional community as to whether plural experiences may predate trauma, and DID or OSDD systems be a specific manifestation of plurality that has been impacted by trauma.
How common is it?
"Systems" as an Identity Term
Another alternate term for folk who do not always identify as Plurals is Systems which also can be used either descriptively (uncapitalized here) or as an identity marker (capitalized here). Using capitalization to distinguish between these is not (yet?) a formal convention in the broader community, and folk may alternate between using these terms as identity markers or descriptors with or without capitalization. Also note that system names (see below) may include "System" in them as part of the group identity.
Systems (as an identity marker) are usually exclusively DID or OSDD systems, systems (as a descriptor) are speaking of their collective internal entities and mechanisms as a whole.
How Large is the overall Plural Community?
There are some who theorize that most people have "people in their head" (e.g. Family Systems Theory), but that they are not necessarily experiencing distress or problems from the phenomenon. Being plural may just be a range of potential human experiences that is much more common than people would like to think. We're told DID is rare — but the generally agreed upon 1-3% of the population-at-large is not really all that rare, is it?