The basic types of AUs.
Some or all of the canon Undertale characters swap roles and/or personalities with the other canon Undertale characters. e.g: Sans replaces Toriel. No new characters are added, and the story is relatively the same for the most part.
If only one character pairing is swapped, it doesn't fit in this type. In that case, the AU is most likely in the Character-centered type.
A mixture of a story, game, movie, TV show, or book, or personal characters and Undertale; A two unrelated universes/shows/etc combined into one AU.
Every single character in Undertale is changed while still remaining in the same basic story in some which way or form. AUs that only affect some of the characters are not found here. AUs that change only during the time Frisk is in the underground are not found in this category.
AUs that are focused on a particular set of characters. These include headcanons, extensions, and flat out new ideas.
AUs in which 1 or more characters are replaced with real people.