CBFC stands for Central Board of Film Censors,Pakistan or Central Board of Film Certification, India. As the question is tagged "Bollywood", I am taking it for the latter one. It is a referred to as censor board for movies in India. The Center Board of Film Certification is a censorship and classification body under the Ministry of Information and Broadcasting, Government of India . Its task as per Wikipedia says is with:
regulating the public exhibition of films under the provisions of the Cinematograph Act 1952. It assigns certifications to films, television shows, television ads, and publications for exhibition, sale or hire in India. Films can be publicly exhibited in India only after they are certified by the Board.
So a certification by CBFC is a must for release of a film in India. The certification of films by CBFC follows.
According to IBN:
The Cinematograph Act, 1952 (Act 37 of 1952), apart from including provisions relating to Constitution and functioning of the CBFC or the Central Board of Film Certification (called the Central Board of Film Censors before 1983), also lays down the guidelines to be followed by certifying films.
Initially, there were only two categories of certificate:
‘U’ (unrestricted public exhibition): Movies such as Stanley ka Dabba, Taare Zameen Par and Roadside Romeo. Movies you would take your younger siblings to watch and not feel red-faced about.
‘A’ (restricted to adult audiences): This category grew to include any movie with expletive language, sex and violence- Kaminey, Love, Sex and Dhokha and Ragini MMS – the slightly off-beat but box office safe movies that the censor board kept away from the children.
Two other categories were added in June, 1983 –
‘UA’ (unrestricted public exhibition subject to parental guidance for children below the age of twelve): The movies like We are Family, Apne, Waqt - the ultimate family and social value films - the ultimate tear-jerkers.
The ‘S’(restricted to specialized audiences such as doctors or scientists).
[above text has been reformatted from original source]
As per wikipedia;
"This rating signifies that the film is meant for a specialised audience, such as doctors."
Now I could not find any list of "s" certified movies by CBFA, but what I really could find is nothing less important. See this annual report by CBFA for the year 2010, no "s" certified movies were made. So it makes clear that "s" certified movies must be hardly made in India. If it had produced in a good amount, we might have found it on Google easily.