The title BBFC stands for ;
Its a board of people that decide whether or not a film should get a specific age rating, they decide if it should get a higher or lower rating.
The ways that media industries identify films are by giving them age classifications, they do this so young viewers are not allowed to see certain things like in a age 18 film you can use sexual nudity, foul language, violent scenes and so on, whereas in a 12 rated film you cant use scenes like that.
In a 12a age rated film you moderate violence is allowed but it should not go into detail. You can emphasis on injuries or blood but not too much.
The difference between 12a and 12 is that 12a is what is shown in cinemas and 12 are what it shown on DVD, TV and so on.
In a 15 rated film it is pretty much a stronger version of a 12a or 12 rated film you can include scenes of strong violence, frequent strong language e.g. the F*%$ word, sexual references, sexual nudity, brief scenes of sexual violence, you can also use discriminative language and work to explore themes relating to this and you can also use scenes of drug taking.