It was easy to ship enemy urbanites and POWs out of CorpEmp territory and into the Cordons Sanitaire. But not everyone actively opposed CorpEmp. For every five communities that welcomed the warlords, there was one that was...apathetic. But not hostile. That was the thing. They just didn't want to join, and wanted to be left alone. CorpEmps founding warlords, not wanting a demographic influx of "people pissed at being made CorpEmp subjects" collectively shrugged their shoulders, and said as one "leave 'em be I guess".
Surveyors (armed in case the locals felt like doing more than shooting stern looks) would drive up to these communities, plop down a fence or whatever means of demarcation that was available, and bam! A Reserve was created. The Reserves (CorpEmp's word them) refers to the many, many, many little rural populations centers that are not part of CorpEmp. Unlike the Cordons Sanitaire, which had heavily policed borders, a Reserve only needed a little guard post (and gift shop for tourists) to watch the country road between it and the closest CorpEmp community. Trade and immigration, in and out of the Reserves, is unrestricted (again, these people were never activity opposed to CorpEmp). The closest CorpEmp and the Reserves ever get to diplomatic crises is cross-border brawls at the nearest pub.
Because of CorpEmp's lax security with the Reserves' borders, there are times were malign actors take advantage for their own gain. Some CPC gangbanger looking to make his bones will try setting up a "honest enterprise" within a Reserve. Days later, some Imperial constable will end up finding their body dangling from a telephone pole by the border.
Reserves are very insular (in case you couldn't already tell), contact with the outside world usually depends on if a particular reserve has internet, and how far the nearest non-Reserve is. Cooperation between Reserves is rare, emphasizing their desire to be self-reliant and not involved with the outside world. Cooperation with the Big Three is even rarer, for the exact same emphases. Although littered across CorpEmp territory, Reserves also form on the border between CorpEmp and U.M. territory.
The following is an incomplete list of Reserves in the 29th century:
Amish Countries: The Amish (the term now includes all strains of Mennonite) have continued their policy of remaining separate from the outside world. A few fellowships have taken this to the level of buying old orbital habitats (even building a few of their own).
Anti-Imperial Tribals: Not every member of the New Tribal Movement in pre-WW3 America was as warlike as the Hispano-Gaels. Some tribes decided to ignore the goings-on in Texas, and wall themselves up. A few still exist on the North American continent.
Bunker Dwellers: In some parts of the world, just before WW3 raged, some affluent individuals renovated old bunkers and missile silos to house themselves, families, and friends. A lot of them decided to remain underground after riding out the war and making contact with the many warlords fighting over the remains. Some complexes have become quite expansive by the 2800s, a few became underground arcologies.
Frontiersman: Some people don't like living in the core of human-settled space, and prefer to live on the fringes. Many communities were established in the Outer Solar System after the Big Three staked their many claims in the Inner, and now that all the good real estate inside the Oort is being developed a few expeditions into the Extrasolar territories (and beyond) were planned.
Hiders: There's not a lot to say about these Reserves. As the name (given to them) suggests, these guys just want to be left alone. In order to do so, they cut themselves off completely from the outside world. Hiders have been making moves to the Solar System's Oort Cloud (expect turf wars with the W.C.O.F.).
Leavealones: The bog standard Reserve. Quaint little village, standard of living a century-ish behind CorpEmp's. Usually a dirt country road connecting it to a CorpEmp community.
Nomads: The descendants of traveling folk, Nomads move up and down the many roads of the world. Some rely on horse-drawn carriages, others make do with the latest and greatest RVs. A few affluent Nomad groups own their own private roads. An even fewer number of the most affluent own their own boats to travel from continent to continent.
Peregrini: "Dwellers around", this Reserve takes the form of an enclave within an urban environ. Usually a walled-off, gated city block.
Prims: Sure, Reserves can be considered luddite by the other (technological) factions, but Prims take it to extremes. Eschewing technology altogether, Prims desire to return to state they only refer to as "monke". Uncontacted peoples, under the jurisdiction of the Green Consensus, are often erroneously placed in this category.
San Marino: Somehow, this small republic inside the Italian Peninsula not only survived the Third World War, but also the Warlords' Wars and creation of CorpEmp. When asked why it was never integrated into the West Latin Macrocommunity, locals just shrug their shoulders and say (in their own language) "I dunno".
Schismatics: Despite being a pro-religious empire, not many religious communities are pro-CorpEmp. Fundamentalist Mormons, Jehovah's Witnesses, Sedevacantists, Orthodox Old Believers, and that's just from the Christian family of faiths. Islamic, Hindu, Buddhist, Shinto, Animist, Scientologist (and other UFO-cults), and other religious schismatics dot the Earth (and the rest of the Solar System).
Spelunkers: These guys are a variant of Bunker Dwellers. What's different from Bunker Dwellers is that Spelunkers (as the name suggests) went to live inside complex cave systems. A bit like Hiders, in that they really want to be left alone.
Steaders: Steaders decided, once the technology was good enough, to strike out on their own on the open seas. Steader Reserves can vary in size, from single family platforms to a whole neighborhood floating on the ocean's surface. Or on the ocean floor.
Survivalists: Hiders, but packing heat. Wait. More heat than Hiders. These guys are playing the long game, waiting out in wherever they hunker down until another cataclysmic conflict destroys the Big Three. Then, them and their vision for humanity will become dominant. Given the recent discovery of multiple objects blueshifitng to the Solar System, they're beginning to be quite popular.