Theory: People are said to have possipoints in http://www.gamesas.com/bgsforums/index.php?s=&showtopic=612570&view=findpost&p=8918365. A point is a mark or something signifying a place. Possi and Impossi suggest possibility. If people have possipoints, and the Convention has an impossipoint, then I'd guess it's a mark, a line in the sand on an infinite desert, which was the dawn era, and a reference by which you can draw the inference of your own possibility. Or it's just an impossipoint because the stone doesn't exist.
I still look at it in the Probablistic manner---
People have possipoints, where there is the highest possibility that they would exists, when taken through the lens of timeless time.
EG, The Orbitals around an atom are Possipoints. The electron has the highest probability of being located in those sections of the universe, in relation to the nucleus. However, there is an infinitely small probability that the electron could be ANYWHERE else in the universe.