As far as I know the term ‘Sui generis’ means a transitional phase of service – neither permanent nor temporary (quasipermanent). It’s an old concept.
I think friends from state govt. services may clarify it in a better way!
Service as a Probationer is “Sui Generis” – neither substantive or officiating. A Govt. employee who is recruited as a probationer with a view to substantive appointment to the cadre of a service or department does not, therefore, acquire a lien on a post in that cadre until he is confirmed in the service or department.
‘Substantive’ appointment means permanent appointment while ‘officiating’ appointment implies temporary appointment to any post for a certain period for which it has fallen vacant because of relieve of an employee on lien to excadre post or for study leave etc.
Burton’s Legal Thesaurus gives us to understand that “sui generis” is a Latin word and it means “in its own category, in its own group, of its own character, of its own class, of its own classification, of its own denomination, of its own desiggation (The choosing and naming of someone to be the holder of a position), of its own genre, of its own kind, of its own nature, of its own type, of its own variety, peculiar, special, the only one of its kind and unique. These types of terms are the flowery and twisted words and are generally used by the administrators in the own interest keeping full power in its hands. I believe I am clear.