As described by Clyde, the Yggsdrasil Event is what is referred to in the future realities as the point from the UnMen’s secondary universe when a Manifesto has become realized. From that point on, the Manifesto spreads out into several possibilities based on slight nuances, similar to the branches of a tree. The metaphor essentially works like this:
Demiurge’s Particle World is the root of the Event. From there on, adjustments were made and a true universe grew.
The UnMen’s universe with the Annunaki is the trunk of the Event. It represents the growth and basis of what would become the further ideas.
After a Manifesto is realized, this becomes the true Yggsdrasil Event. Several parallel realities are formed as a result of possible Manifesto’s but unless one is realized, all of these branches are basically quantum universes. And until the trunk determines its future, all the branches are both valid and invalid possibilities, some with the potential of backtracking as the Entity has done.