Fission
The initial set of primary basic subjects (PBS) included in a scheme for library classification results from a division/fission of the UoS in a manner similar to the division of the UoS by scholars among themselves as convenient fields of specialisation. It may not be possible to discern a specific characteristic used in this initial division.
Fission is the process of division or splitting or breaking up into parts. This process has, until recently, been denoted by the term "dissection". However, dissection usually implies the splitting, breaking up, etc., of an entity into parts by an outside agency. On the other hand, fission is an internal process of division without the involvement of an outside agency.