Post-Neutral Sequel AUs are AUs that take place after one of the Neutral Routes of Undertale.

The exact cut-off between an AU qualifying as a Special Event, and an AU qualifying as a Post-Neutral Sequel is Frisk leaving the Underground. In Undertale, after defeating Omega Flowey, Frisk must go through the northern door for the AU to qualify as a Post-Neutral Sequel. If an event happens before Frisk is able to leave, then the AU is a Special Event.