Basic Change AUs are AUs that have some fundamental aspect from Undertale or, in the case of AUs of AUs, the base-AU changed in such a way that everything is affected. Whether that be where all the Monsters are instead gods (like Reapertale), where all the Monsters are violent, fell creatures (like Underfell), or where the entire world has been altered from the world of Undertale or the base-AU (like Dreamtale), they are all Basic Change AUs. AUs where all the characters are the same, or the same character from multiple universes take up most of the cast are also Basic Changes.

Story Overhaul

Story Overhaul AUs are AUs that completely change the foundation of the story of Undertale or, in the case of AUs of AUs, the base-AU in such a way that it still clearly resembles its inspiration, while its premise is disconnected from it (like Reapertale).

Theme Overhaul

Theme Overhaul AUs are AUs that completely overhaul the general theme or "feel" of Undertale or, in the case of AUs of AUs, the base-AU. This can either be simply aesthetically, such as Outertale, or alter the characters' personalities, such as Underfell. The base plot is kept mostly unaltered.

World Overhaul

World Overhaul AUs are AUs where the world of Undertale or, in the case of AUs of AUs, the base-AU is completely overhauled to the point that there is barely any resemblence to the world of Undertale or the base-AU.

AU Where Everyone is the Same Person

An AU where everyone is the same person is, as the name suggests, an AU where a large number of characters are the same character. This doesn't just include AUs where all characters are, for example, Undyne (like Undynetale), but also AUs where a bunch of the same character from many different universes are present for the story, and take up a majority of the cast (like Christmas Party).