The Rim

The Rim is a vast but very sparsely populated region on the edges of Alliance territory, with only very few true/planetside colonies. Most of its inhabitants - the Rimdwellers - live aboard spaceships or spacestations, and small mining outposts on moons and asteroids. Because the region is so massive, sparsely populated and home to very few strategic locations, the region has been mostly ignored by Alliance governments. The region isn't represented well in things like senates, and it hasn't been acknowledged as an independent to the ire of some of the Rimdwellers. All in all, it's become somewhat of a lawless frontier: there are plenty of civilians there who just want to do their jobs and live out their lives, but there's many outlaws, pirates and scavengers there too. Many of the Rimdwellers are upset with Alliance government for ignoring the region, which is why many Rimdweller ships are hostile towards Alliance vessels.

General description
Most Rimdwellers are just civilians with no bad intentions, but the Rim is far away from the rest of Alliance civilization, and authorities have little power there, making it the perfect place for pirates who want to be close enough to the Alliance to be able to raid colonies and convoys, while staying far enough away to not get caught.

There are some Aegi-settled areas that aren't part of the AFU, but there's not many of them and all of them are rather small. Many colonies in the Rim aren't officially affiliated with the AFU anymore, and consider themselves independent states.

It's close enough to the Alliance for some manner of trade, and sometimes minor raids on Alliance convoys by Rim pirates, but far enough away that the Alliance can't afford to always accomodate for the area.

Most Rimdwellers visit some Alliance colony at least a few times in their lives, though they often stick to the outer colonies rather than the central planets. Children are often sent to the outer colonies (or the central planets) for schooling, and many Rimdwellers have friends and family living outside of the Rim. They also trade with the outer colonies, and the mining operations are usually run by central planet organizations so there's supply lines going from Alliance space to the Rim and back as well. There's plenty of Rimdwellers whose job it is to transport goods along these supply lines, so they travel to and from central Alliance space regularly.

The Rimdwellers would claim abandoned colonies and such to attain new resources and set themselves up as a new force on the galactic scene, being angry at the Alliance for having been largely ignored thus far.

There are a lot of mining colonies there, as well as spaceships of various sizes and purposes, but no large settlements or densely populated planets. The region is so far from the main Alliance worlds and the major colonies that the Alliance doesn't do a lot of peacekeeping there, and the region is largely ignored by Alliance politics due to the distance, the vast scale of the region and the relatively small population spread over that massive expanse of space.

Currently largest Rim settlement is a vast mining colony in Fentat-A Iros. The living conditions there are decent: most facilities are present, including a small hospital, (private military) law enforcement, basic schools and more. The standards of living aren't the same as in true Union colonies, but for the Rim, it's a paradise.

The Rimdwellers are seeking for the Alliance to acknowledge the Rim as its own independent state, but that request has thus far failed to get any sort of traction in the rest of the Alliance. Understandably, many Rimdwellers are fed up with the Alliance, and some have even grown hostile to the rest of the Alliance. That hostile faction is only a fraction of the population of the Rim, however: there are still plenty of loyal civilians in the region that just want to do their jobs and live their lives without worrying about politics and hostilities. That makes the situation in the Rim even more complicated.

To many Rimdwellers, Alliance laws and restrictions feel more like oppression. Plenty of outlaws in that region that don't care for AFU laws and traditions, so the more powerful/wealthy people there are likely to use technologies like for ex. weaponized prosthetics.

Military
It doesn't have any centralized military force. Its defences are as strong as the defences of whatever colony you'd decide to attack, which is not very strong in most cases. There's a few colonies (mostly mining colonies that extract very valuable materials) that have decent defences set up, but many are practically unprotected safe for some firearms and maybe a turret or two.

Most Rimdwellers are actually just citizens and so are unarmed, though the percentage of armed people in the Rim is vastly higher than in the Union and the rest of the Alliance. Many Rimdwellers are miners and so have some tools on them that can be used as weapons, and as there's not much of a central law enforcement, it's easy for citizens to acquire weapons if they want them too.

The cybernetic augments used in the Rim are mostly things like mechanical limbs (loss of limbs isn't uncommon in the more dangerous mines of the Rim), but also some other enhancements like cybernetic eyes which can see into a wider spectrum or can zoom in and out. Many of these augments are a bit iffy, and are prone to glitches, since the R&D in the Rim is limited. Some of these technologies actually come from within the Union or elsewhere in the Alliance, but since those technologies face a lot of restrictions in the Union it's usually leaked or stolen military experimental technology, or something built by a rogue group.

Architecture
Rimdwellers also have a slightly darker colour scheme compared to the rest of the Alliance, and look more rusty/poorly maintained as opposed to the clean visual style of the Alliance.

They'd fit into a bit of a cyberpunk aesthetic. Most of their larger settlements are spacestations and the like, and those have a bit of a scrappy aesthetic combined with cyberpunk to them. Not much nature. it'd be clustered mixture of different racial architecture blending with neon lights. It's a very big and relatively diverse place. There's some parts of the Rim - especially the more populated colonies closer to central Alliance space - which are at least post-cyberpunk, and might even have some of the solarpunk elements of the rest of the Alliance/Aeginian Federal Union. The most backwater parts of the Rim sit very much within cyberpunk style, while the majority sits a bit in between, likely very similar to post-cyberpunk styles.

It'd be something like a superstructure with winding busy streets, lost of rimsteel variations of racial buildings, dotted with small pockets of traditional building materials.

Union trains are generally on rails, though many of them are magnetically levitated as opposed to having wheels on the rails. They do still follow rails though. There are difficulties in keeping a smooth running schedule with all the shuttles flying around.

The metal used by Rimdwellers isn't allosteel, but a lower-quality ferroverium alloy. Allosteel is made of ferroverium, iron and carbon, with very minor amounts of some other minerals. Rimsteel is also a ferroverium alloy, with a lower quantity of ferroverium, more iron, some carbon and also copper and zinc (which creates the signature colour of Rimsteel). The components of Rimsteel are more readily available in the Rim, and the production processes are less reliable so there's more variation in the quality of the metal. Allosteel is an incredibly fune-tuned alloy, while Rimsteel is more makeshift.

Fashion
A lot of casual clothing in the Rim is similar to that in the Union and the Alliance, but they have developed some styles of their own too. This often includes some re-purposed work gear, like construction or mining equipment. Most of the settlements in the Rim are spacestations and asteroid bases, but there are a handful of planetary settlements too. These are generally built on moons and other celestial bodies with a lot of resources, but these are often not very suitable for life. They have the advantage of having natural gravity, but you still need to wear protective gear at almost all times unless you're inside a protected habitat. These planetary colonies are usually quite small too, with small populations similar to those on board the spacestations and asteroid bases.

Residents
The majority of Rimdwellers are Aegi, but there are people of all (Alliance) species there. Like the Trink, Hymid, Jorgasians and Notix. In rare cases there are a few Narvakhan (Thelean exiles), though they mostly keep to themselves and avoid other inhabitants of the Rim. There's various other species to be found there too: there's some Akkimari there, along with Cratons, a few Avikan who fled the Nomada and even a few individuals of species that the Alliance hasn't even had any official contact with.

Aegi
On average, they're nearly the same. There's definitely differences in their areas of expertise, but Rimdwellers are usually just as intelligent as other Union citizens. The two groups also aren't fully seperated: there's plenty of Union citizens moving into the Rim, and vice-versa.

The Rim doesn't have any central education system, but then most (Aeginian) Rimdwellers grew up in other areas of the Union. The Rimdwellers that are born in the Rim usually either get home-schooled, go to a small local school in the Rim, or go to school in the Union or some other Alliance nation.

The inhabitants of the Rim are still citizens of the Alliance and its member nations, but the region is mostly ignored by Alliance politics. People of the Rim are free to move in and out of Alliance space, but they're upset that the Alliance won't give them the same benefits that people in the inner territories get. Education, healthcare and many more things that are common in the Alliance aren't anywhere near as good in the Rim, and the Alliance isn't spending time or resources on improving the situation in the Rim.

There's definitely resentment there, though it differs from colony to colony. Some Rim colonies are actually pretty well off, while others are very much an afterthought. So long as they are officially part of some Union state, its inhabitants are guaranteed an equal income, but the infrastructure of these colonies is often lacking and it's difficult to purchase things out there. So while the individuals technically get the same wealth, there's fewer ways to use it (and it's often more expensive), and at the same time colony infrastructure and services are lacking.

Many mining colonies export a lot of resources to the central Union territories, and while they do get their 'fair' income in return, little of the state's resources are spent on improving those same colonies. There have been a few cases where colonies have broken off from the Union, but these don't often end well as most colonies are still reliant on the Union for many of the resources that can't be obtained locally.

Trink
The Trink on board Rimdweller ships are definitely the crew's appointed engineers. Aeginian engineers are good, but Trink are generally better and more reliable. It's also why you find the Trinkian NPCs in the back of the ship, near the Hyperdrive: they're the ones maintaining and calibrating the core.

Rimdweller ships are generally either equipped only with sub-FTL engines (for colonial mining ships and such), or with more basic Hyperdrives like the ones found on the in-game ships. Union ships have a neatly contained core that's both safe and reliable. Most FTL-capable Rimdweller ships make do with simpler cores that aren't as reliable. Their ships sometimes have to reboot the core a number of times before a successful jump can be made, and the simpler sub-FTL engines don't provide the same thrust making the ships slower to accelerate.

Most incidents with a Hyperdrive core simply result in the machine turning off before the gate is opened. In extremely rare cases, a core might overheat, and an explosion could occur. What happens then depends on the core: if it's a well-built core with proper shielding, the blast could be contained within the core and you'd only lose your Hyperdrive. A core that isn't as well protected could cause the explosion to rip a hole in your spaceship.

Even rarer are incidents in which the drive malfunctions while the gate is being opened. That could result in the Hyperdrive core (and some of the surrounding space) being ripped from your ship and plunged into the Hyperspace dimension, leaving you with a big hole in the middle of your ship.

Trivia

 * [Asked about object called 'rimbackgrounddoor'] "Those screw-like pillar with the gears on top are the primary locks for the door, they're what allows those doors to stay closed even under massive stress. They spin, releasing the lock connecting the top and bottom pillars, then they retract into the ground and the ceiling to make way for whatever's passing through the door. Retraction is on the Y axis, spinning axis depends on what model you use. Some game engines, for example, use different models for rotation so it could be X on one and Y on the other. "
 * Starbound doesn't currently support NPCs attacking player's ship, but if that functionality was there the Rimdwellers would be one of the factions that could attack it.
 * Friendly Rimdwellers are planned to make an appearance. There are plans to add Rimdweller spacestations that are inhabited by both hostile and friendly Rimdwellers, and player would get some new quest, crafting recipes etc. from those friendly ones.
 * Modern FTL/hyperdrive ships don't generally need the people on board to take anything, but some older sub-FTL ships do. They use constant acceleration to simulate gravity, which is fine so long as you don't exceed 1G by too much. But some of these older ships were built to accelerate even further, and people on board such ships need some special drugs and the like to survive the extreme forces and stay conscious. Such propulsion ships are still used sometimes, especially when budgets are small such as in the Rim. Those ships can only really be used for interplanetary travel though, as they can never cross the interstellar void in a reasonable timeframe.
 * Briggs is one of the primary companies doing construction in the Rim! They're usually at the front lines when a new mining colony is established, for example. The Briggs Guardians are especially useful in the Rim! Within the more populated Alliance space, Briggs usually works within cities and well-established space docks, where there is little need for additional security. In the Rim, however, there's a greater chance of running into pirates, hostile aliens and other threats, and the Guardians are much more useful since a M-ASA escort isn't always possible. The Guardians can provide on-site security when official military and police aren't available.
 * The Rim has a decidedly bland cuisine compared to that of Elithia.