Post-AU Sequel AUs are AUs of AUs that take place after the story of the base-AU.

Due to the diverse nature of AUs, each cut-off between an AU qualifying as a Special Event, and an AU qualifying as a Post-AU Sequel is to be determined on a case-by-case basis by the Undertale AU Wiki's Staff.
Baseline content looked for in determining if an AU is really a sequel of another AU is that the entirety of the content from the base-AU must be canon, and the AU in question must be taking place after the end of the base-AU. The base-AU must also actually have a story to set the sequel after, so making a sequel in this fashion of something like Underfell isn't possible, since no story has been released by Vic. However, if the base-AU is more along the lines of just Undertale, but different (like Underfell, Underswap, or Altertale), then the general cut-offs found in the other three Sequel types will be used.