WHAT IS A SYSTEM?
A system is the de-incorporation of a single selfhood/mind into multiple different parts, all present in the same body.
As a disorder, it was previously known as Multiple Personality Disorder. As of the DSM5 it is now categorized as Dissociative Identity Disorder (DID) or Other Specified Dissociative Disorder (OSDD). It's characterized by the seperation of the self into different parts or segments, each usually equipped to deal with specific tasks or scenarios that the other parts of the mind are ill-equipped to handle. These parts may or may not have amnesia barriers from the other parts, may experience loss of time, and the sense of a loss of control of their body. Oftentimes alters/parts will request to be known by different names, pronouns, and have different behavioral traits than other alters. As a disorder it is very understudied, but is most commonly theorized to originate through severe or long-term childhood trauma (usually occuring before the age of 12). Alters are created via the process of "splitting", which is usually caused by stress (but can be caused by other situations)
It is a covert disorder usually diagnosed later in life.