Limbo (or Abyss) is the term used for the transmundane layer that exists to catch things that fall through the cracks of the world.Others lurk inside limbo, as well as people who have had their connections to the world completely severed or when whatever harvests the soul of the dead refuses to take them and drops them in. It is also known as the Abyss to Others.
A lot of Bogeymen are born here. It is later confirmed that their anger and hatred eventually boil over and allow them to surface. It still has a hold on them, however, and can take them back. Practitioners who fall through the cracks no longer have their powers, but retain their knowledge.
There are many theories to how Limbo came to be. Eva states that it is the ground-level of the world, where it all began. Zoey Artana theorizes that each manifestation was once a demesne, and it was attached to some vital process of reality. Through this vital process, it came to devour other demesnes and objects, and it swelled in size. It connected to other such areas, and formed the backbone for what might otherwise have been the original void.
Void was an endless storm of everything under the sun, a great elemental grinder to churn up all which fell into its reach, but over time it became more civilized as other demesnes and Gods also fell into Limbo when forgotten or the ones who maintained them pass on, becoming part of it and determining the rules. The gods sleep in the walls and bring logic while demesnes bring memories of their masters.
Despite the nature of Limbo, it fulfills an essential need the universe has, the need for healthy entropy.
Also known as the Sewers or Compost Heap, it is a manifestation, where the sky above is nothing but sheer darkness that has a weight and presses down, making you want to crawl in the mud beneath your feet. Bits and pieces of things from the real world could be found there too. It's a constantly evolving maze that's only growing as more and more things fall through the cracks, but the different parts are connected to one another via tunnels and small lights.
A manifestation of steep vertical surfaces that look like apartments, the air is thick with choking dust, fog, and darkness. The bottom disappears into darkness and fog for a good kilometer or five, red and orange lights around that flicker violently. The windows are spaced too far apart, or clustered, and took different forms as sections of building had fallen and somehow jammed together, like a game of Tetris. Anything that looks easy is a trap and the hand holds are riddled with insects, broken glass, or other obstacles, while the insides are nests for Others and Bogeymen.
A manifestation that occurred when the Hillglades House fell through the cracks, stairwells and bookshelves lay along every wall, books filling them as water and dirt flowed in along the sides, streaming along the surfaces, turning what had been conventional novels into sodden messes. This manifestation of the abyss is aggressive, actively targeting and attacking those within it and shifting around at inopportune times, changing the landscape. The books are either empty or hold things you don't want to meet. Anyone making noise within the library is subsequently attacked by the Others or a giant set of hands.
Inside any manifestation, certain rules hold true, such as cardinal directions. You are what you eat and lose your sense of Self. Its entire purpose is to break down whatever falls in and, if you want out, the only way is to go through by changing, adapting, eating, and getting stronger. It forces you to deal with either a manifestation of your past (shadow of memories), present (visions), or future (dreams), usually one or the other and the forms can vary, but never normally all three.
When attempting to have someone conform using traumas of their past, it may manifest as a black version of some aspect of their memories. These solid entities aren't actually Others, but parts of the people in question that they have been trying to run away from. Most people flee from it, others kill them and essentially get rid of the memories that composed them in the process, but it is possible to accept them.
Likewise, when sleeping the person can see a possible future and then take actions to confront or reject it, or rid themselves of the need for sleep to abandon them as well. A person with visions of the present can choose to ignore them or run away from them as sometimes it is easier to remain in Limbo rather than face what awaits in reality, but they can back trace their steps to receive the full message and learn what they need to do.
Regular spirits are practically nonexistent in the Abyss. Instead the Abyss relies on something spirit-like but it's colder, more detached, and tends to only follow its own rules. Generally it gets brought in or leaked from elsewhere, thus often manifesting as a leaking effect. It also has an insulation effect that makes whatever it covers gradually change into something more difficult yet also resistant to change from outside factors. 
Practitioners attempting to communicate with the Abyss for favors will have to do so with a personification of the Abyss. Since the Abyss is quite hostile and abrasive, the personification will be as well. It's likely that a time limit will be placed during negotiations, with death traps being set up around them and Others converging on their location.
By cooperating with Limbo, it provides an avenue of escape should the person in question be able to reach it. However, it will still have a hold on them and can pull them back.
Visiting places where it’s closer to the real world than others where you must:
Conform: Cooperating with the forces driving Limbo in some way, you become sensitive enough to possibly find a way out. Although most who do no longer desire to leave.
Adapt:Green Eyes stated that in one part, you have to swim against a current for a while without a chance to surface, meaning you must change enough to resist the current and breathe underwater.
Become powerful enough: Some parts are guarded by really strong things that eat you if you're too weak or make you agree to carry something back.
Have someone visit Limbo and pull you out: Practitioners can open a way by using a circle and sacrificing something valuable to it that they will never get back. This is the only way to get back to the material world while maintaining your sense of Self, but practitioners usually only seek out the worst monsters since finding someone specific is difficult and not worth the cost of entering. 
Become a Bogeyman: Arguably a subset of the above options. These denizens are filled with rage and malice, Bogeymen bubble up into the material world.
“You go where anyone goes, if they slip through the cracks.“ [...] “It changes from place to place, urban area to rural. But it’s the spiritual equivalent of rock bottom. It’s where people like Dowght might go or be chased off to by locals when the imp is done with him, or where things like the goblin you fought might dwell, if they weren’t quite strong enough to hold a territory. Dark places, dog-eat-dog, unpredictable, hard to navigate. The spirit analogue to the deep wilderness.” [...]
“Sometimes there’s shit,” she said. “Sometimes fire, sometimes a garbage dump or a lightless pit or it’s a frozen wasteland without any light to go by… like I said, it varies from place to place. It’s a place defined by the misery and self loathing and desperation of those who dwell in it.” - Excerpt from Conviction 5.3
↑Wildbow: On the layout of the Abyss - yes, it's all interconnected. Blake (and Green Eyes) starts out in the Outskirts, very close to the Drains. Had he taken another path, he might have made his way to the box store, or into the edge of a city/urban Abyss area. Or he could have gone further into the woods and ended up in a place similar to where Midge went.
Who you are, how you go, and where you are when you go determine where you end up. You're not going to go to a place where you'll immediately think "Oh, I'm in the Abyss"- it'll be similar to where you were when you dropped in. If you pass through from a wartorn area where people are lost, desperate, and trying to survive, you're going to find yourself waking up in a place where the city is all ruins, everything's burned, and 'bandits' are prowling around - and it steadily becomes apparent the bandits aren't limited to very scarred/mutated humans. Or you could end up in a series of buildings that are perpetually on fire.
By this metric, Blake goes from an area of wilderness of the sort you find on the periphery of a big city, trash strewn with hints of civilization or outlying buildings, to the outskirts. Except, you know, horrible.
When Hillsglade House fell to the Abyss, it connected to some similar regions, tapping Others more fitting for the Library than the School (Schoolmarm), Manor (Diary Girl), etc. Normally areas of the Abyss don't form so blatantly, but more as places are forgotten or lost.
Some places will be lost or abandoned (say, a small town struck by natural disaster and neglected by neighboring states or countries) but won't become Abyss so much as they trend that way over time, with more and more monsters and horribleness appearing over time, as Others find their way in (where there's a great deal of easy prey and easy living, until competition gets fiercer) and the Abyss gets more of a hold. Places can be pulled out of this 'sinking' state, but it's hard.
You can die and get ground down in the abyss, that route is available, but it's hard. If you fall from the tenements you're liable to land in another area with all limbs broken, but still be alive, as things come after you. If you get eaten alive, you might get partially torn apart, partially digested, and shat out with some semblance of life. There's also a whole tract of things in the Abyss that collect living things, whether it's to dress them up like a doll and add to a collection or keep in cages, to snip pieces of for later snacks. They do this because the life and emotion of living things is serves them like a campfire might for you or me. So, you know, you can die and probably will get ground away (or transmuted into something nonsapient) eventually if you're not very tenacious, but actually getting there is hard and it's fraught with peril of the 'now I'm in a cage for the next decade' kind. - Finished Pact a couple of days ago, quick review and questions regarding the Abyss inside (Wildbow, Reddit.com, 2018-05-29)
↑ 3.03.13.2are there other potential uses for this portal?
For example, if the practitioner can also pass through the portal, there are probably a significant amount of non-quite-bogeyman and other unfortunate ex-people who would love to negotiate for a way to exit
Yes. But you just sacrificed something valuable to get there. One could argue that overall, given a longer period of time, you're probably going to break even or fall short when it comes to finding useful Others and people in the Abyss. Some of them will backfire on you, because even if you're not specifically selecting Bogeymen, you're picking people who are at least a few steps closer than normal to being Bogeymen. By and large, you're probably only going to get more than your money's worth if you're interested in violence and terrorizing or if you need very desperate individuals for something. Think in terms of like, "For this job, we're going to have to limit ourselves to the ex-con homeless" somethings. Filling out the roster with ask-no-questions, doesn't-matter-if-they're-pretty people for your out-in-the-middle-of-nowhere pyramid of sacrifice? Good stuff, go shopping for your dredge cultists in the Abyss. Bank robbery? Probably not, unless the 'robbery' is more on the bloody side.
If you're good at negotiating and finding talented individuals, you might do better than break even for a while, until you get burned a few times, at which point you're only breaking even.
Or, if you're a Nomad/City Mage, could you communicate with the Abyss for favors and whatnot?
You can communicate. For favors? Well, you'd be talking with a personification/representative of the Abyss, and the Abyss eats up and digests people and spits out monsters. You're not going to get a wholly civil conversation here, and it's not necessarily going to be on a level playing field. So... you open a door or tap a door you've already made, you venture into The Abattoir, and you find a clearing big enough to open a discourse with the embodiment of that specific region of the Abyss. Chances are this conversation is going to happen with a very hostile, intractable, abrasive personification of this very hostile, intractable place. It may happen with a time limit, a horror movie deathtrap slowly setting itself up around you as you try to establish terms and talk, or it might be that Others are on the approach, reacting to unconscious signals from the Abattoir personification- the Ram-headed Man, the Milkmaid, the Calfbone Child, and the Font of Meat converging on your location while you talk.
Is it doable? Yes. But probably for very specific purposes. More often it'd be to broker passage or establish a place-as-an-ongoing-ritual scenario, one that would involve feeding the Abyss. More of an endgame play for a specific scenario than something one would set up for later gains.
Are there even regular spirits in the Abyss, and if so, are they subject to the "cosmic in-sink garbage disposal" effect?
Not regular spirits. Something spirit-like is in effect, but it's colder, more detached and self-referential, and tends to follow its own rules. Brought elsewhere or allowed to leak elsewhere, it would manifest in leaking effects from the relevant area. Corrosion, persistent ulcerous rust, bloodstains where there shouldn't be any, and an insulation effect, where things become both changing and stubbornly resistant to outside factors - spirits would have a harder time affecting a door that refused to close properly or lock, that creaked loudly as it swung open and closed, banged violently at the slightest brush of wind, and seemed to get more troublesome over time... but spirits would have a harder time affecting it. To an extent, you'll have a harder time affecting it too... this means connections to these things may not form, they may slip out of one's possession very easily, or not get any easier to use with time. Of course, that gets easier if one becomes more like them, meeting them halfway.
Around areas where the Abyss is closer to the surface (which happen to be areas which are less linked in to the rest of the world... and areas that are less likely to call news stations and say "Hey, I've got this weird thing!"), these things tend to find a way in. It's usually a harbinger (though not Harbinger) that a specific place is on the way out. People and animals can collect these traits too - family generations with more mutations or stubborn health issues (physical or mental), that somehow prove very long-lived and more and more detached from the rest of the world. Scourges may collect or extract that essence for use or fashioning crooked things, but it's a tricky process that requires more of a biopsy and then nurturing of the biopsied part, or a more complete extraction, but then with a 'it has no connection to you so it slips from your grasp' limited time as the metaphysical pocket has a hole in it and that essence is steadily lost over time until spent. - More Abyss Questions (Wildbow, Reddit.com, 2018-10-23)
↑“We need to sacrifice something of value, destroy it irrevocably, and say a few words.”
“Define ‘value’,” Peter said.
“If you have to ask, it’s probably not valuable enough,” Alexis said. - Excerpt from Duress 12.5