T'au Septs
Since their lack of astropaths means that messages and proclamations can take the equivalent of Terran weeks even to reach across the T'au Empire, the T'au have gotten pretty good at delegating. With each new wave of settlement (or "sphere"), T'au designate worlds that are most suitable for settlement as "sept worlds." These serve as the local center of government for the system and sometimes other systems nearby. Fittingly for the naïve space communists, it is roughly equivalent to an SSR.
Each sept has a certain number of worlds within its borders, and each sept has its own dialect, fleet, and army, as well as having its own way of waging war.
Minor Note
It should be pointed out that these septs are divided into two unequal groups: the Empire proper, which is located near Ultramar; and the newer, weirder, Nem'yar Atoll, a region of Imperial space somewhere near Baal. The last three septs on this list are in the Nem'yar Atoll, while the rest are located in the Empire proper.
It's worth noting that videogames and campaign books mention a few other septs, but they didn't make the cut to the 8e Codex, so presumably they're either isolated from the rest of the T'au or have been abandoned/overrun by enemies.
Also only five of the septs have official rules but there are now rules for "Custom Septs" which can be used for your own fictional sept or just one of the minor non-canon septs.
T'au
Obviously, the homeworld gets its own Sept. The Capital of the T'au Empire, it is heavily defended by Orbital Stations, significant Kor'Vattra assets, and even a massive fortress on the planet surface to protect the Ethereals in case of invasion. The planet itself has a few seas, with arid Savannah and scattered mountains. T'au from this sept are considered especially learned and wise.
The only other couple of facts worth mentioning are that no alien has ever set foot on this planet, and that T'au has a moon named Lu'val, which they colonized, and that most new colonies base themselves upon this sept.
In acknowledgement of their devotion to the Greater Good and the patiently defensive behavior usually advocated by Ethereals, their Sept Tenet in 8e gives them 5+ in overwatch and when using the Greater Good ability.
T'au'n
The first colony the T'au ever developed, this sept and its planet specialize in incredibly complicated plans as well as naval combat. They have the largest Air Caste stations and host the greatest number of Kor'vattra ships. T'au from this sept are considered pioneers and spacefarers.
Interestingly, this sept highly advocates the idea of having multiple weapons on a battlesuit, instead of just strapping on three melta guns and calling it a day.
D'yanoi
A tidally locked world that was separated from the Empire by unnatural space storms. As a result, they fell behind the rest of the Empire, and are considered rustic and backwards. Basically T'au rednecks.
Fluff indicates that D'yanoi was home to Reptilian beasts that would eat the colonists, indicating that it is very much close to a death world, and that Orks tend to roam the area, though they generally get their butts kicked by the bluies. They also tend to have an abnormally high numbers of Vespid allies in their Cadres due to the sept's proximity to the planet Vespid.
Bork'an
Probably one of the more prominent ones, this sept is the big research sector of the Empire. Warriors from here have fantastic weapons and are generally restrained in attitude. When training to use the KV-128 Stormsurge Battlesuits (aka BFG attached to legs), T'au will be sent here, where they must graduate from the academy in order to be allowed to use the Stormsurge with their partner.
To reflect their cutting-edge technology/craftsmanship, their Sept Tenet in 8e gives an extra 6" of range to their Rapid Fire and Heavy weapons, making their gunlines just that much scarier.
Originally, a race known as the Poctroon inhabited the planet, but they died off due to a plague, and the T'au took the planet after their species went extinct.
Dal'yth
This planet is where the T'au ground the Damocles Gulf Crusade into a stalemate. Originally, Dal'yth was a planet comprised originally of lush jungles and segmented beasts, but was forcibly tamed by the T'au until it essentially looked like Shanghai in speeeehhhhssss. Essentially, Dal'yth was OG T'au Vietnam.
As a result of all their time hunting in jungles and ambushing Ravenguard and Catachan, the T'au of Dal'yth have become excellent at camouflage and ambushes. This is reflected in their 8e Sept Tenet, the benefit of cover as long as they stand still.
This extensive colonization sort of wrecked the native population, and now the planet is home to a pretty wide and diverse variety of xenos along with Water Caste T'au, who serve as cultural mediators and facilitate trade with off-worlders.
Dal'yth is also the birthplace of Commander Puretide
Fal'shia
Fal'shia is the Tau version of a Forgeworld, but its sept is devoted to it. Fal'shia T'au are considered great problem solvers, and are the ones who generally fiddle with technology until they make it better than it was before. The Ad Mech could stand to learn a thing or two from this sept, if they weren't too busy blowing gaskets over the amount of tech HERESY present.
Also of interest is Commander Or'es'Ka-led warriors from here in the Kaurava campaign. Fire Warriors from Fal'shia willingly test new weapons, even if it means they explode on their face and kill them, and otherwise closely resemble the Bork'an in demeanor.
Vior'la
Vior'la is the T'au version of Tatooine (T'autooine?). Colonists have to live in special domes because the planet is ravaged by plasma storms, and is in fact the way they once defeated an Ork invasion. T'au from here contrast with Bork'an mostly by being the hot-blooded anime protagonists. Breacher and Pathfinder teams tend to be popular here for these reasons. Vior'la is the Birthplace of Farsight, and is home to some of the most prominent Fire Caste academies.
One of the more derp stories for the T'au involve Vior'la and some World Eaters. Somehow, the influence of Khorne was able to catch on with the Vior'la Fire Warriors, convincing them to break gunlines and enter close quarters combat with the World Eaters. Obviously, this cost the T'au very dearly. Some Ethereals were dispatched to deal with this, but the fact that this even happened is fucking stupid. Though Khorne does seem to take an interest in T'au from Vior'la, so... it’s stupid anyway. While T'au are not immune to Chaos—including a freaking Chaos God turning its gaze on them—this was often in individual T'au, not entire armies.
Still, despite this possible Khornate influence, their 8e Sept Tenet nonetheless focuses on shooting rather than melee by letting all their Rapid Fire weapons count as Assault, and removing the penalty for advancing and firing said Assault weapons. So while they don't do any better in close combat, they do at least get there faster, for what it's worth.
The 9th Edition Codex expanded on their aggressive tendencies further: Vior'lan implementations of the Patient Hunter philosophy overwhelmingly use crippled or cut-off hostile forces as a lure, and even the sept's Water Caste diplomats have a reputation across the Empire as something akin to a used car salesman.
Sa'cea
Sa'cea is a highly urbanized sept and the last sept of the First Sphere of Expansion. Before the world could be colonized, the T'au had to land forces to kill off flesh-eating predators. So in that regard it appears they learned from the D'yanoi affair.
Packed to the gills with more kids than they know what to do with, Sa'cea T'au sent off a ridiculous number of colonization fleets during the Second Sphere of Expansion. As a result, they're generally seen as stoic and honourable warriors.
On the tabletop, 8e a Sept Tenet that buffs their Leadership and lets them re-roll one hit per unit because they're just that cool under pressure.
Au'taal
Au'taal is basically a massive retirement villa. T'au from here tend to be regarded as lazy: this sept is where old heroes of the Empire would retire to if they survived long enough. Aun'shi was probably gonna end up here before he was reassigned to keep kicking ass, much to his chagrin.
Au'taal has a big fucking pile of guns, because defending old people is important and shit, and the fruits harvested from here tend to be used to trade with other races.
N'dras
N'dras has a moon, and an Ocean World, named Kvariam, which was invaded by the Space Wolves at some point. N'dras is considered a spooky and strange world, and most T'au don't like it, considering anyone from there to be untrustworthy, brooding, and quick tempered. Earth Caste scientists often require hefty protection from the Fire Caste when conducting experiments here.
It was all-but-abandoned by Ethereal decree, to serve as a secret testing location of the XV95 Ghostkeel battlesuits.
Kel'shan
Tau from the Kel'shan sept were one of the first attacks from the Dark Eldar arrived here. Then, they met the Death Guard. Oh, also the Imperium. Aaaaaand the 'Nids.
It used to have two other colonies in its borders, Sh'draig and Ka'mais, but both got nommed by the nids. Regardless, the sept has held on pretty well, able to beat them all back.
Essentially proto-4th sphere xenophobes, the surviving sept members are solemn and openly hostile and mistrustful of aliens. So, basically they’ve achieved something close to what passes as common sense in Warhammer 40,000.
They also spearheaded the development of the XV107 R'varna Battlesuit.
Elsy'eir
A T'au sept equivalent of theatre kids to Bork'an science nerds and Kel'shan DIY lunatics. They're generally seen as artistic and intellectual compared to the more applied sciences of other T'au worlds.
It has many moons, most of which are mined for valuable minerals to produce both regular battlesuits and the powerful XV104 Riptide. Production facilities are located here as well, making this a valuable region for the Empire.
Tash'var
A sept that have some of the best Razorshark pilots and Breacher teams in the Empire. T'au from here are said to be hardy, tenacious, and courageous. They have handled Ork invasions and raids, much like Kel'shan (but with less xenophobia).
They also have precious gems which they use to trade with other races in the galaxy.
Vash'ya
The last sept to be founded during the Second Sphere of Expansion, it was a barren world that was terraformed by the T'au to make it breathable again. The Air Caste settled into zero-G orbital settlements right away however and still make up a pretty sizable chunk of the sept. The vast network of orbital infrastructure over this world makes it an ideal hotspot to recruiting spaceship officers and crew.
Once, the Hrud invaded this world, but the T'au beat them away with some drones and remote control battlesuits. Why the T'au don't use these more often is a question which GW haven't yet seen fit to answer.
T'olku
T'olku is essentially a model sept designed to demonstrate the awesomeness of T'au'va to potential allies. As one would expect, it is the place where aliens are sent to learn about T'au culture. It also has large Ethereal temples, and was once visited by an Imperial Fist, who was there to try and delay the T'au from invading the world of Nimbosa through sneaky diplomacy.
As one would expect from an Imperial Fist, they failed hard.
Ksi'm'yen
Formerly an Imperial world, it was conquered by the T'au, and Gue'vesa who were here were evacuated to the middle of the Empire for their safety. Generally, considering what the Imperium does to defectors, this was probably a good call.
Hilariously, the T'au captured a genestealer, then decided to experiment with it, until eventually a rebellion broke out, resulting in a decade long quarantine before an eccentric Ethereal declared the planet clean and productive in a move that is definitely not going to end badly (if GW ever remembers to progress this particular plotline).
Fi'rios
A former Ork world, these planets were wrested from them by the legendary Shas'O Kais and is a veritable fortress. However, fighting still occurs, but these T'au refuse to give up due to their tenacity and all out pure ballsiness that puts them leagues above other T'au.
Mu'gulath Bay
Destroyed during the Third Sphere of Expansion. Not much worth noting, except that the capital got an Exterminatus dropped on it, only for an Earth Caste shield generator to protect the T'au and their allies inside. Many of the survivors attempted to defect to the Farsight Enclaves, but most were killed by loyalist T'au forces.
Mu'gulath Bay was the last—and greatest—failure of the Third Sphere of Expansion.
Fe'saan
It is described as a beautiful world in the Nem'yar Atoll, and is home to most of the T'au administrative operations in the region. The Water and Air Castes maintain significant assets here, the first mainly to spread propaganda about the Greater Good throughout the Atoll.
The world was first colonized by Survivors of the Fourth Sphere of Expansion, and later further developed by Fifth Sphere T'au.
Due to their horrific experience with those prototype Warp drives, they've become very different from most Tau: They're more violent, sadistic and highly xenophobic to the point of going apeshit when they meet auxiliaries and other Xenos, especially towards Gue'la, whom they attack and kill on sight unless stopped by other Tau. Needless to say, a bunch of rabid blueberries wouldn't do wonders to the Tau's image in the Atoll, so the more rowdy ones are removed from public view and reintegrated by force if deemed necessary.
Kor'tal
Another Death World that the T'au colonized. This sept specializes in producing and collecting dark matter from an orbiting black hole which is used in industrial and military assets, the most notable of these being the reactors for XV104 Riptide Battlesuits.
Yo'Vai
A curious sept that literally has a giant fucking Halo ring, except that it is just a landmass with a breathable atmosphere. The Earth Caste continues to debate what it is and how it formed, though the most widely accepted one is that it was some ancient terraforming technology.
Fire Caste tank pilots and Air Caste pilots from around the atoll usually train here.