Caste is a system of social stratification characterized by controlled marriage, hereditary transmission of a style of life which often includes an occupation, ritual status in a hierarchy, and customary social interaction and exclusion based on cultural notions of purity and pollution. Its paradigmatic ethnographic example in Halcyon is the Empire of Hel, a society divided into rigid militaristic, religious, racial, and social groups, with roots in a mythological ancient history involving Ur as told in their monotheistic religion, Helena's, primary text, the Golden Ratios.
On Halcyon, caste categories like "presented gender," "sexual orientation," and "species" exist only within the walls of Hel, though there are parallels in the Underworld's Life Roles which are held strictly to biology.
Aureans believe themselves descended from a race of blond, white-skinned elves called Uryans, haling from the Disgraced Empire of Ur. Their solar deity Hel and its associated practices, themes, and teachings are all visually consistent with their saying, "Light Makes Might." The concept of purity, alien to most of Halcyon and anathema to witchblood, is prized among the Aureans. "Species", so-called, are arranged in a hierarchy by skin tone and mythological Uryan elfin features rather than by actual racial separation (i.e. a white-skinned elf and a white-skinned human are treated as components of a singular Aurean Species descended from Ur, while kunkaar and humans of any skin darker than pink are referred to as having "nyx-blood", "nyx" being their derogatory term for Urmen). Despite this, many Hellions with darker features choose to mimic the alleged golden skin of the Old High Uryans of their mythology by staining their skin orange over their darker features. Others with any darker features who can afford it will bleach their skin using chimaestry.
Genders are likewise ranked, with males-presenting-as-male at the top, followed by Aurean presenting male, then Aurean women, then nyxblood men, then nyxblood women, and then all asexuals or shifters like Dreamenders at bottom. The lightest skinned, blondest men, therefore, are considered to be inherent leaders and the chosen of Hel. At bottom are female "nyx" (Urwomen) and female "nyx-blood" (dark complected peoples and peoples who tan easily are considered Hel's accursed). Special dispensation is made for those who alter their appearance or sacrifice aspects of their own labels to better match the Uryan ideal. These are called "conversion caste," and there is typically a glass ceiling on the boost in standing one is granted.
All other groups outside of the Empire are considered apostates and thus caste-less.
The Kunkaar Imperium and remnants of the so-called Ur Nation coalesced into a collection of egalitarian matriarchies where pregnancy and birth are exalted, men are relegated to non-leadership positions, and only intersex beings are allowed to choose social placements. However, because marriage is not controlled, there are no economic classes, and there is neither emphasis on nor any comprehension of the notion of purity, Underworld Life Roles are still easily distinguished from that of Hellion Caste.
In the Evening, where most beings dwell, most languages lack gendered pronouns, there is no such thing as race (people organize around common lifestyles rather than genetic patterns--but common genetic traits thence evolve), and there are no militaries, just diverse, agrarian towns or villages dispersed throughout.Among Eveners and humans, caste is considered a metahex and a primary, necessary feature of "pyramid" (i.e. hierarchical) societies, designed to mystify people into shuffling themselves into positions in the pyramid so that an agent may better control them and filter their available knowledge. It's the subtle poison of the Auro, one that traps you into a game of sacrifice and slavery in its name, all under the promise of paying one's way to freedom. To an Evener, money is serfdom. This is why instructions to Hellions are to arrest any grouping of more than three Eveners, because it is said they form natural covens and bewitch any who listen to their nonsense words.