Here are Scott P. Scheper's top-level branches:
- 1000 Arts & Humanities - 2000 – Social Science - 3000 – Natural Science - 4000 – Formal Science - 5000 – Applied Arts & Sciences
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SCHEPER, Scott P., 2022. Antinet Zettelkasten. San Diego, CA: Greenlamp. ISBN 979-8-9868626-0-6, p. 284
The process entails searching the Wikipedia page of the Outline of Academic Disciplines for a given field. From there, you simply choose a number arbitrarily under the parent branch from which to build out a subject area. Material related to life self-development and life philosophy can be added to a section within Applied Arts & Sciences, for instance (5411 – Self-Development).
Cf. Concept object page 
(OpenAlex API documentation)