A split is what happens when a systemmate branches off and separates from an existing one. This may be (but is not always) a traumatic process.
Splitting, from a medical and psychological perspective, is most commonly thought to occur when traumatic memories, thoughts and ideas are unable to be integrated into the core sum of who a person is, resulting in the eventual formation of a new identity (1). Despite this, some may identify as having split yet not being the resultant of a traumatic event.
1. ^Alpha, Rose, et al. “DID Research.” Splitting, DID Research, did-research.org/did/identity_alteration/splitting.html.