related, but not drawn from as I've been thinking about the continuum of taxonomies and subject headings for a while...
On the Spectrum of Topic Headings in note making
Any reasonable note one may take will likely have a hierarchical chain of tags/subject headings/keywords going from the broad to the very specific. One might start out with something broad like "humanities" (as opposed to science), and proceed into "history", "anthropology", "biological anthropology", "evolution", and even more specific. At the bottom of the chain is the specific atomic idea on the card itself. Each of the subject headings helps to situate the idea and provide the context in which it sits, but how useful within a note taking system is having one or more of these tags on it? What about overlaps with other broader subjects (one will note that "evolution" might also sit under "science" / "biology" as well), but that note may have a different tone and perspective than the prior one.
This becomes an interesting problem or issue as one explores ideas in a pre-designed note taking system. As a student just beginning to explore anthropology, one may tag hundreds of notes with anthropology to the point that the meaning of the tag is so diluted that a search of the index becomes useless as there's too much to sort through underneath it. But as one continues their studies in the topic further branches and sub headings will appear to better differentiate the ideas. This process will continue as the space further differentiates. Of course one may continue their research into areas that don't have a specific subject heading until they accumulate enough ideas within that space. (Take for example Daniel Kahneman and Amos Tversky's work which is now known under the heading of Behavioral Economics, a subject which broadly didn't exist before their work.) The note taker might also leverage this idea as they tag their own work as specifically as they might so as not to pollute their system as it grows without bound (or at least to the end of their lifetime).
The design of one's note taking system should take these eventualities into account and more easily allow the user to start out broad, but slowly hone in on direct specificity.
Some of this principle of atomicity of ideas and the growth from broad to specific can be seen in Luhmann's zettelkasten (especially ZK II) which starts out fairly broad and branches into the more specific. The index reflects this as well and each index heading ideally points to the most specific sub-card which begins the discussion of that particular topic.
Perhaps it was this narrowing of specificity which encouraged Luhmann to start ZKII after years of building ZKII which had a broader variety of topics?