It began with the movie studies and moved into the book trade. It has no legal basis and will not stop someone suing you in fact it might even make things worse as the plaintiff argues that it is based on my life and to rub salt into the wound they claimed it was fictional. It does get into hilarious situations such as a book whose main character is a well-known figure from history and the disclaimer says "This is a work of fiction. Characters are not based on any person, living or dead." I've read that the disclaimer was sent up wonderfully by the Three Stooges who had as their disclaimer to a historical piece "If this movie bears any resemblance to persons living or dead it would be a ****** miracle."