Not a necessary system, but a system of essences.
People are believed to be members of a caste because they have "essence" of that caste.
Their physical being is in some way linked to their caste membership. People may "pass" but cannot change their essences.
Next: Race and caste
Essentialism in this sense (in Western philosophy) dates back to Plato and Aristotle. The linguist George Lakoff talks of the "essential" as the thing that "makes the thing what it is, and without which it would not be that kind of thing." But this doesn't really get at the issue. In the doctrine of varnas and in many other cultures, human are thought of as having group essences. For example, people are not Dalits because they look a certain way or have certain behaviors. Rather, they have behaviors and are (falsely) believed to look a certain way because their essence is Dalit. If they did not have those behaviors or didn't match the stereotype of what Dalits look like, it would make no difference: their essence would still be Dalit. Dalits face persistent discrimination and sometimes violence in India. As large numbers of Indians have immigrated to the United States, they have, in many cases, brought their prejudices against Dalits with them. In a 2018 survey of Dalits in the United States two thirds reported that they faced caste based discrimination in the workplace, 41 percent said that they experienced discrimination in education, and a quarter that they had experienced physical violence. All of this takes place in areas and industries that have a high Indian population and is largely invisible to those of us who are not members of the Indian Community. Numerous tech companies including Apple, Amazon, Alphabet, and Dell have specific policies against caste discrimination. In 2023, Seattle and Fresno CA outlawed caste discrimination. In 2023, the California legislature passed a law outlawing caste discrimination but it was vetoed by the governor (who argued that it was unnecessary since castism was already covered in other anti-discrimination laws).