Stories that fall into this definition are usually what-ifs, where possibilities arising from different circumstances or character decisions are explored. Unlike regular fan fiction, which generally remains within the boundaries of the canon set out by the author, alternative universe fiction writers like to explore the possibilities of pivotal changes made to characters' history, motivations or environment.
The author gets an established audience for their story: the fans of the original, and then subsequently altered, universe, which they would not get if they wrote it as an original story instead of fan fiction. Some of the best fan fiction writers, who aspire to be published authors, can take advantage of this inversely by developing a loyal audience, the readers of their fan fiction, for books with original storylines that they might publish at a later date.
This universe was created after fans first saw previews of the Disney film, Frozen. Fans saw the two princesses, Elsa and Anna, and immediately found connections in their physical appearence to Jack and Rapunzel, identifying them as potential children. Elsa is a beautiful queen to-be who possesses ice powers like that of Jack Frost. Because the film gives no explanation as to why Elsa posesses her powers, fans quickly sought to fill in the blanks for themselves. Also, Elsa's exotic platinum blond hair looks close to Jack's frosted hair.
Anna is the younger sister of Elsa and is brave and a confident young girl like Rapunzel. Anna's strawberry blonde hair resembles that of Rapunzel's golden locks. She also posesses similar traits to Rapunzel as well as life experiences, being locked inside her home and unable to truly explore the world. Many fans have sought to create a story where the four characters interact and develop, focusing on the Frozen storyline occuring well past both Rapunzel's and Jack's own personal film experiences.