The Only Use

caste system
in
To Kill a Mockingbird
(Edited)

There was indeed a caste system in Maycomb, but to my mind it worked this way: the older citizens, the present generation of people who had lived side by side for years and years, were utterly predictable to one another: they took for granted attitudes, character shadings, even gestures, as having been repeated in each generation and refined by time.
p. 149.2

caste system = a system of social ranking

Definitions:

(1) (caste system)
a rigid social system that divides people into ranked groups, with very little chance to move between groups; especially the traditional caste system in India based mainly on birth

In traditional India, this system divided people into hereditary groups that often decided whom they could marry, what kinds of jobs they could do, and how others treated them. Today such discrimination is illegal, but caste-based prejudice still affects some people and communities.
(2) (meaning too rare to warrant focus)