The principle of atomicity is a governing idea of the zettelkasten method of notetaking and knowledge management.
The principle suggests that each note should address a single unit of thought—that is, that each note card contains just one idea. To paraphrase Andy Matuschak, notes should be about one thing, but capture the entirety of that thing.
Decomposing a text or concept in this fashion helps decontextualize individual ideas and consider each one on its own merits. As well, keeping notes "atomic" facilitates the creation of links and connections between different ideas in ways that would be difficult with traditional notetaking, e.g. if multiple ideas are collected in one document. Atomicity promotes re-use of notes, in turn creating more opportunities for linking and connection.
But, notes should not be so narrowly focused that they become fragments. If notes are too granular, they may lack context. Be kind to your future self: write notes so that they are completely understandable without need for additional references.
Ahrens, Sönke. How to Take Smart Notes: One Simple Technique to Boost Writing, Learning and Thinking – for Students, Academics and Nonfiction Book Writers. Sönke Ahrens, 2017.
Matuschak, Andy. “Evergreen Notes Should Be Atomic.” Andyʼs working notes. Accessed October 10, 2020. https://notes.andymatuschak.org/Evergreen_notes_should_be_atomic.