v154 · symbols()
In counter() and content
A counter style is not only for list markers. counters() flattens a whole nesting depth into one string, and symbols() can supply the style it counts in — so a document outline can be numbered in whatever notation the document uses, without naming anything.
A numbered outline
The rule
counters() — plural — walks every counter of that name in scope and joins them with the separator, which is what turns three nested lists into b.c.a rather than just a. The counter style applies at every level.