Once someone leaves their mortal body, the essence of them goes to the next plane, or better yet, planes.
Being received in the Ideal Gate, they quickly learn they need to complete one essence course to leave to the next part of their journey.
From there, they are free to choose where to roam: The Realm of Presence, Absence, Accretion, Depletion, Infliction, Reaction, Harmony or Duality. Truthfully, they are absolutely unbound to do any of the courses, being allowed to change mid-path and even try to master all eight.
Each Realm is a reflection of the essence, which truly sounds marvelous but due to mass not existing here and the laws of physics being less followed, things go alien very quick.