Palimpsest universe

Savar, when exposed to Omega's mastery over the Agathon Equations, discovered the true nature of the Prime history as existing atop infinite overwritten palimpsest universes. While each history believed that the past was unalterable, forces with Agathon Engine level power could rewrite time, making new histories which replaced the old ones and consigned them to the margins of existence. As a whole, this meant that N-Space was "riddled with paradox and contradiction likes weevils in a biscuit." (PROSE: The Infinity Doctors)

History
In the overarching span of overtime, there existed a history of history's writing, rewriting, and overwriting. (PROSE: The Gallifrey Chronicles) A Faction Paradox boy suggested that the very first palimpsest history had been one where the Doctor was a human from some planet in the 49th century, fleeing from an Enemy who'd overrun his home. The boy suggested that the Enemy had overwritten this history when the Doctor wasn't looking, creating new universes where he'd always been a Time Lord, and eventually ones where he'd always been half human. (PROSE: Unnatural History)

After the Disappearance of Omega, Omega found the anti-matter universe allowed him to watch the rewriting and overwriting of history from a neutral vantage point. In every single palimpsest, events still occurred which caused Omega's disappearance, ensuring that he stayed unchanging. Among these palimpsests were one where there was no Rassilon and the Time Lords only had Omega as their founder, (PROSE: The Infinity Doctors) as was the case during the First Omega Crisis; (TV: The Three Doctors) one where Rassilon still ruled Gallifrey from deep within the Matrix; and one where Rassilon was a woman and Omega's lover. Omega learned to control this overwriting process using the Effect, allowing him to create new histories at will. (PROSE: The Infinity Doctors)