The characters in the original work have new stories written by the same author, or the world outlook is still the same as the original work, or the world is simply an overhead world, just the characters in the original work.
Fan books are divided into fan picture books and fan novel books. There are many such things in Japan, even in Taiwan Province Province and Hongkong, but they have just started in mainland China. There are many people with the same culture or map, but few people can really see it in the market when they are compiled into a book.