Borders are a hell of a thing in three dimensions. They aren't lines, of course. They aren't usually even walls. If anything, they're planes where spheres intersect.
I made a decision about how my FTL drive works that kept the whole "space is a lot of big empty nothing" front and center-- namely, you're still navigating in real space, you've just created a field around your vessel in which reacts to normal space in an amplified way. So if you're going from Earth to Alpha Centauri, you still have 4 light years to traverse, just you can do it in, say, three weeks instead of twice as many years.
What this means is there's a lot of space to "control", once a civilization has decided it wants to hold dominion over a region of space. What even is a "region" of space?
I have to confess something: I really loathe when sci-fi has some area of space defined as "Sector 47" or such, because it seems so utterly random. What is a Sector? Why is that one "47"? I like a bit of sense and order to these things. I like the idea that they were designed by someone who had a system.
So I had a system. Actually, two, in a way, but the same root beneath it, that root being a Cartesian coordinate mapping system. I prefer Cartesian coordinates to the Right Ascension/Declination system*. An X-Y-Z grid, marked by light-years, creates a clean system similar to latitude and longitude. So, since this is a human system, Earth is the "Greenwich", at point 0,0,0.** Thus, Indus Colony, for example, has the coordinates (5.66, -3.16, -9.9).
This system breaks all of space into eight Divisions, based on where they are, positive or negative, on the X-, Y- and Z-axes. Initially, I went with Greek letters-- Alpha to Theta-- to name the divisions, but A. that struck me as to close to Trek's "Alpha Quadrant" and such, B. offered potential for confusion, since the FTL system also used the Greek alphabet. So I took a different form of classical, with the eight divisions being: Zeus, Hermes, Gemini, Poseidon, Athena, Artemis, Apollo and Taurus. It has a certain degree of arbitrary to it, of course, but human naming systems can be arbitrary from time to time.
Next, I broke those Divisions into Sectors and Regions. A Sector is simply a cubic light year, defined by its Divisions and Cartesian Coordinates. So Indus Colony is in Sector Taurus-6-4-10. An alien colony, further away, is Paxin Gamma, (9.82, -7.78, -27.33), and it's in Sector Taurus-10-8-28.
But when you're talking in terms of space, a cubic light year is nothing. Traveling from Indus Colony to Paxin Gamma takes you through 18 sectors, and there really isn't anything there. Some of those sectors are clearly Human controlled, some are Paxin controlled, and some... aren't much of anything. So, where is the border between Human Space and Paxin Space? Is it defined, or is there a no-man's land somewhere between?
So, Regions give something with a little more scope, though they are only 1000 cubic light years. "Only", as if a 10x10x10 ly cube was something to sneeze at, but again, in an interstellar scope, that's still zip codes on an global scale. But it gives one an area of space that is easier to define, and define "ownership" of. Taurus-111 is clearly Human, for example, while Taurus-113 is Paxin controlled. Taurus-112, in between them? That's more disputable...
The other system divides the neighborhood into Expanses-- which are 30x30x30, aka 27,000 cubic lightyears. Expanses are kind of the Celsius to the other system's Fahrenheit. It still uses Cartesian, and uses eight division, but it just numbers them 1-8. Then each 30-ly block is letter-coded. Expanses aren't as useful for figuring out, say, borders or areas of control-- that Indus-to-Paxin Gamma trip is all in Expanse 7AAA-- it's helpful for figuring out larger geopolitical interactions. Sectors and even Regions are rarely populated by more than one species. Looking at Expanses gives you a better sense of how they bump into each other. But even that can be daunting-- in my defined 150-ly radius sphere, there are over 600 Expanses.
So that can give you some idea how big the "big picture" really can be.
*- Though I'm given to understand that RA/D is preferred by astronomers.
**- Which it is on Star Trek as well, despite the fact that the Federation is supposedly formed by many species. Earth is still the center. Hmmm.