Web fiction
Alternate universe
A fan fiction set in a deliberately divergent version of a story's world: the AU departs from canon while keeping the characters.
In fan fiction, an alternate universe (AU) uses a parallel universe "as a setting for a story that departs from the canon of the fictional universe" the work is based on; such stories "are usually referred to as 'alternate universe' works, often abbreviated to 'AU'." Wikipedia sorts them into role-swaps, which exchange places between characters, and new-setting AUs, which move the cast into a different world. The stated motivations are exploring alternative narrative possibilities and "fixing" plot points a fan disliked.
For a writer the AU is a portable tool, not just a fan fiction custom. It is the cheapest controlled experiment in character: the cast stays constant while one variable, the setting or a single event, changes. Serial writers run the same play as side stories and spinoffs, and an AU branch is worldbuilding homework too, since each deliberate divergence from canon must be chosen and tracked or the story accumulates contradictions.