The following article contains spoilers.
Please read at your own risk.
Not much is known of how the caste system in Illéa started, although it was hinted that people's castes were based on the amount of money their family donated to the government during the first war with New Asia. When a character is first mentioned, there is generally a note made of what caste they are in. Eight is the lowest caste and One is the highest in The Selection Series.
It is said that when a female gets married, they will take the caste of their husband. This is why Aspen did not want to marry America in the beginning of the story: he did not want to force her to move lower down the caste system, to a Six, and suffer what he was suffering.
In the end of The One, Maxon Schreave is named King when his father was killed by Southern Rebels; he had been making plans to take down the Caste System, and he described to America that it was going to be a long process, starting with the Eights merging with the Sevens and so on.
Carter and Marlee Woodwork had the honor of becoming the first casteless citizens before the abolishment of the castes was even publicly announced. Thus, their son Kile was the first child in Illéa who was born without a caste.
By the beginning of The Heir, the castes had been completely removed for a few years. The Illéan people actually rejoiced and celebrated about the removal of the castes, but King Maxon and Queen America then faced conflicts on the provinces because the labels were not removed in people's minds.
Nowadays, a lot of family names in Illéa reveal the previous caste of the family. Baden Trains probably comes from a family of Seven. Tavish Plowholder's and Winslow Fields's families might have been farmers and Paisley Fisher's were probably fishers, thus Fours. Harrison Driver's father might be a driver, thus a Six.
Royalty and religious figures; Maxon Schreave, his family (King Clarkson, Queen Amberly, and Adele) or just royalty are Ones. After marrying him, America Singer and all her family members also became Ones. Maxon's and America's children (Eadlyn, Ahren, Kaden, and Osten Schreave) would be Ones too if the castes still existed. A position of power and respect and the very top of the caste system.
Celebrities; including athletes, singers, actors/actresses, models, politicians, police officers, firefighters, guards, and military members. Celeste was a Two and was a model. Aspen, formerly a Six, is also a Two and is a guard at the palace. After marrying him, Lucy also became a Two. Officer Avery and Officer Markson are palace guards, so they are in the Second caste, too. Carter Woodwork, a previous Seven, became a palace guard and thus was upgraded to a Two. When Carter was caught with one of The Elite, Marlee Tames, they were both caned and became the lowest caste, Eights. She married him and, with Maxon's help, they were both secretly upgraded to Sixes as palace servants. Both of them, however, eventually became attendants of King Maxon Schreave and Queen America Schreave and the first citizens without a caste. Natalie Luca married an actor and became a Two before the caste system died out. Hale Garner is also the son of a basketball player, so he would be a Two if the castes were still in place. Some Twos were known to be snobs, like Celeste Newsome for the better part of Maxon's Selection.
Businesspeople including inventors, teachers, philosophers, scientists, doctors, veterinarians, nurses, dentists, architects, librarians, engineers, therapists/psychologists, film directors, music producers, lawyers, and writers. Girls that are or were part of the Selection are also part of this caste, unless they are a natural Two or already a Three. Kriss was already a Three. However, America became a Three when she was chosen for the Selection.
Jewelers, real estate agents, insurance brokers, head chefs, construction managers, property/business owners, and farmers. America's older sister, Kenna, and her husband are factory workers. Natalie, Anna, and Emmica are from a family of farmers — all of them were Fours before the Selection. Elise was Four before being selected, although her parents are actually very rich owners of luxurious hotels. Marlee and Queen Amberly were also Fours before their respective Selections.
Artists and Performers; including stage actors, classical musicians, singers, dancers, photographers, sculptures, and circus performers. In The One, Maxon confesses that he would be a photographer and therefore would be a Five with America, if he wasn't a prince. America's family (Shalom, Magda, Kota, May, and Gerad) were Fives. America's older sister, Kenna, was formerly a Five before marrying James, a Four, and becoming one herself. America was also formerly a Five before entering the Selection and becoming a Three, later on winning and becoming a One, her family as well.
Workers; including secretaries, waiters/waitresses, housekeepers, seamstresses, store clerks, cooks, and drivers. Anne, Lucy, Mary, Mitsy, Paige and possibly Sylvan Santos are Sixes. The servants at the palace are Sixes. Aspen Leger was a Six, too, until he started working in the palace as a guard. He then became a Two.
Manual Laborers; including gardeners, construction workers, farmhands, gutter/ pool cleaners, and "trash collectors, ditch diggers, movers." When signing up for the Selection, America notes "a girl, obviously a Seven, still in her work clothes. Her muddy boots might not make the picture, but the dust on her overalls probably would. A few yards back, another Seven was sporting a tool belt." Carter Woodwork was also a Seven before he became a guard. Sevens are very similar to Sixes, but they have outdoor jobs instead.
"Untouchables"; including mentally/physically unwell people, addicts, runaways, illegitimate, and the homeless. In a deleted scene from chapter eight of The Selection, it was written that Eights are "people who had been abandoned or orphaned with no way to prove their caste. And then people who fell into heavy drug use, the mentally and physically handicapped, they were all unemployable and left in the streets to beg." America and Maxon met a homeless Eight called Paige when they left the palace at night to meet the Northern rebels. Traitors to the crown are also in this caste; Marlee and Carter were considered traitors as they had an affair, but Maxon forgave them and secretly gave them jobs in the palace, making them Sixes. Instead of them, he sent 2 other people, who were siblings, away to see their family. Eights lived horrid lives until Maxon became king, abolished the caste system, and helped those in the lower castes. Usually, when wealthy people become eights, they are killed and left in the streets. The Eights are the lowest of the lows, and people were punished by being made an Eight.
The Northern and Southern rebels have no caste, but they would most likely be considered Eights as they might be considered "runaways." Marlee and Carter are the first two people in Illéa seen with no caste.