As someone who received a professional diagnosis of DID in 2017, I feel like it's important to say that my diagnosis didn't really do anything to change my experience... while we fit the DSM criteria for dissociative identity disorder, at the end of the day all that really does is let my doctors know that I'm not "normal". Lol.
You absolutely do not need a diagnosis, nor do you need to fit any diagnostic criteria, to be multiple. While you do need to have experienced trauma, experience time loss, amnesia, and so forth to fit the criteria for a dissociative disorder, you don't need to experience anything but having numerous people in one head to be multiple. It doesn't matter what make and model they take.
There's no one true way to be plural. :-)
On the subject of links, Azaphaer provided a great resource. I think Kinhost
is great as well- the name can be confusing for people coming from our community XD they've got nothing to do with otherkinness, but rather, they've been a massive resource for plurality for many, many years. we've personally been using them as a resource since about 2013. They have loads of content to read through, we highly recommend.