And to hijack my own example.
I might create that extra table, then think about what i’m doing with it - if the enrolled students in a class are “all the members of the section”, then i’ve created a redundant table - I can already determine which students are enrolled in a class by following the link from Class → Section → Student (or Person, or whatever you call that table).
If they’re not ALL of the members of a section, then I need some way to differentiate between the students that are and aren’t enrolled in my class.
(I, like Dave, am playing in the dark here, having been educated in the US. This system sounds eastern-hemisphere to me.)