The Spiral Politic was a term often used in describing the events of the War. According to The Book of the War, the best simple definition was "the parts of history that matter." It was essentially a metatemporal perspective on spacetime, in which worlds could be mapped by "importance."
The Homeworld was at the core of the Spiral Politic, the "eye" which observed history. (PROSE: Spiral Politic) Of course, the Homeworld's position as the thing which defined "existence" or "importance" through observation was up for debate, as the enemy also had some kind of equivalent. (PROSE: The Beginning of the War (a Chronology))
Scarred worlds were those which had previously been time-fronts, but had suffered so much confusion that they were scarred beyond strategic value. They included the sites of the Thousand-Year Battles: Kaiwar, Utterlost, and (presumably, though little can be said of it for certain) Mohandassa.
Remote worlds were the erratic, unpredictable locations under the influence of the Remote. The Remote had pirate time technology, and so couldn't be pinned down like the majority of lesser species worlds. This unpredictability motivated the Houses to destroy them during the Second Wave. (PROSE: Spiral Politic) When the Second Wave severed the Remote from Faction Paradox, though, it meant that they were divorced from linearity which potentially exacerbated their unpredictability. (PROSE: Tobin) Ordifica, although destroyed, was an important Remote world. Fallahal was the only one that could be considered stable, as a Broken Remote settlement. (PROSE: Spiral Politic)
Frontier worlds were somewhat similar to posthuman worlds; they were those that existed past the "frontier in time." In the region beyond that point, the future held so many time-active cultures that it was too chaotic to be charted, or to be useful in most ways. Zo la Domini and Ascension both existed almost exactly on the frontier.
Officially, only the Yssgaroth have intruded into the Spiral Politic from outside, but even they are arguably only created by the puncture, not something with a true origin in a separate universe. (PROSE: Spiral Politic)