The writing up of the temporal cosmology in the book is neither detailed nor complete (and even outright whimsical in places), but certain things can be concluded from the bits scattered throughout the book. In general, the time model is presented in a way consistent with a single malleable history, where any change to the past will erase the initial iteration of the timeline, as exemplified by this statement on page 75:
Thank you for killing us all in a bright flash of arcane physics so that we
may live again in a world that knows no such horrors.
It is also preceded by a statement that time travellers are becoming ontological orphans, i.e. the changes they do have no retroactive effects on themselves, and thus after travelling back they remember a previous iteration of the timeline, not the current one:
Any changes you make in the past will
affect the future, possibly even preventing some important events in
your life. That’s okay. Your subjective past does not change, even though
you’ve essentially annihilated your old timeline in favor of the new one.
This is also confirmed on page 78:
THE FUTURE CANNOT OVERWRITE THE PAST
From the observer’s point of view, the future exists only as potential.
Subjective time isn’t affected by events that haven’t happened yet. Even
as time moves forward and probabilities collapse into certainty, the past
of the observer is unaffected.
Also, as per Cycle of a Timeline (starting on page 220-235) a degree of rubbery historical inertia is in play, though probably more akin to river water flowing around a rock: making changes to history does lead to reshuffling of some things, but they still 'strive' to lead to the same end result in the far future, at least initially.
While advocated from a dissociative perspective, the book nonetheless leans towards a chaotic-time-like idea that a timeline should not be revertable to its previous state (page 225):
DON’T CHANGE THINGS BACK
When you change a catalyst, don’t walk it back to its original state. Why?
Because the fun of the timeline is changing the timeline.
What seems a bit unclear is whether time jumps are done as a single unit even when multiple people jump together, and whether changes propagate throughout the timeline 'instantly', or at some Achron-style 'delay', allowing multiple jumps from the same iteration of a timeline into an earlier point within it. Page 233 gives ambiguous evidence that either a delayed (or, more trickily, gradual) propagation is in play, or single-unit jumps are possible, or both:
Early in a campaign, especially during and immediately after the first
adventure, the people sent back after the heroes will be from the same or
a very similar future. Keep these NPCs mostly the same, with only small
differences. Convey that the future has only started to change.
There is evidence that propagation speed has a ratio of approximately 30:1 or higher, since it seems the future humans can occasionally send someone back for the sole purpose of updating their agents about what the current situation is like, on page 77:
We might send someone back to check on you periodically. We won’t
know if there are changes, but you will, so after they brief you on what
the future is like, you should have a good idea of what’s happened.