DID, meaning Dissociative Identity Disorder, is a dissociative disorder often caused by trauma (Traumagenic). DID causes split mind, function, and personality within a body, leading to different alters or system members. When having DID, one is a system. A system's main purpose is to help the mind continue living and manage daily life after repetetive traumatic events. DID is often claimed to be caused by intense trauma, but from what I've learned from other systems and my own research, it's actually caused by repetetive trauma.

To be plural means to have multiple present personalities. One can be plural without having a dissociative disorder, this is often simply a single person having multiple self contained identities, not splitting their mind but rather changing how they act situationally. It depends on the person for why they would be plural. When having DID you are plural. When having DID you have multiple personalities present to an outside perspective, thus, you will appear to be plural. Most people who are plural have a dissociative disorder (DID, OSDD, UDD), but as I mentioned, not everyone, just the majority. So when someone says they're plural, they are likely referring to having a dissociative disorder.