Reference #6: How To Take Smart Notes
The Zettelkasten uses an index to provide entry points for discovery. Next to carefully chosen keywords are the numbers of a few (two to three) cards which relate to that keyword.
When assigning keywords, as a writer you should ask not how to store your ideas but how you would retrieve it. You should write keywords so that you stumble upon the notes under the right circumstances.
Ahrens. How To Take Smart Notes, 2017. (109-110)