Sectors are a mapping convention that started with the older Republic, so they predate the rise of the current Revanchist Republic and the Bern Monarchy altogether. However, it's been sufficiently useful that it's taken over almost all of known space as a convenient way of referring to geography, much the same way that on earth we tend to all use Mercator projections and the Julian calendar, no matter what culture or nation you're from. This sector is a kind of frontier sector, which the great powers see as ripe for further colonization and exploitation, as well as an outlet for aggression against each other that is unlikely to lead to a greater hot war (although they may yet find that this paradigm is not true.) This sector is called the New Alderamin sector, named for the star Alderamin in the Cepheus constellation. As I've said before, the tone of this sector should resemble a space-faring "Scramble for Africa" in most respects (or perhaps the scramble by European powers to settle and colonize the Americas a few centuries earlier); Great Powers (and Lesser Powers) all rushing to claim and colonize a frontier region, establish priority to trade routes, exploit native resources, etc. before their rivals can do the same. Many worlds will be relatively lightly settled in terms of population, because it's only been a few centuries since settlers first arrived on them. Other worlds had native populations that either had to be coexisted with or pacified.
That said, even the Great Powers have ancient history in the area; subsequent Dark Ages have had them lose much of their former heritage, and some of the attention is attributed to trying to rediscover sites that were culturally, archaeologically, religiously, economically, strategically or politically important in the days of the semi-legendary Marian Empire or even earlier—like 19th century British or Prussian or French archaeologists discovering the ruins of Troy, or finding sites related to the Romans, or the tomb of an Egyptian Pharoah. It's not totally the discovery of new frontiers; it's also the re-discovery of their ancient heritage, to some degree.
The subsectors don't have names (contrary to Traveller conventions) just numerals. There are four rows horizontal, labeled with a Roman numeral, and four columns each labeled with an Arabic numeral. The top left subsector is labeled I-1, therefore, while the bottom right subsector is IV-4. The following grid illustrates the layout.
I imagine that the both the Monarchy and the Republic are mostly located to the "north" of this sector; the Monarchy more to the West and the Republic more to the East. The Monarchy's "southern" frontier will therefore be seen at very tops of I-1 and I-2 and the Republics on the top of I-4, mostly. The Empire, on the other hand, is to the East and much of the eastern portions of II-4, III-4 and IV-4 are actually it's western Marches. The Outremer Imperial group is, as the name suggests, separated geographically from the actual Empire, and is located in near the upper right hand corner of IV-1 and a few surrounding parsecs of the neighboring subsectors.
Those are the Great Powers, but the Dhangeti Cartel is located almost entirely within this sector (probably centered around II-3) and a significant chunk of the Cilindan Arm is within this sector too, especially in II-1 and II-2. All of the other sector "regional" powers will have to be developed later as part of the star mapping process. So, of the planets I've already created, the following will go in the the subsectors as noted.
This gives me a very high level gross geography to start working with. The bolded and color-coded labels make it a little easier to spot at a glance.
- Capital Publius—not in sector; located to the galactic north of I-4 at least several subsectors away.
- Dimidium (Prime and Secundus)—not in sector, located due north of I-2 almost an entire sector.
- Yuggoth—II-3, but very close to the bottom left corner.
- Tars Dhangeta (and Bruttium)—II-3, not too far from Yuggoth.
- Phovos Mal—not in sector, would be located to the east of III-4 at least a good sector away.
With that done, now I can get to work on the real task of mapping the sector!