Zettelkasten notes are atomic and autonomous.
Per David Clear:
The principle of atomicity: The term was coined by Christian Tietze. It means that each note should contain one idea and one idea only. This makes it possible to link ideas with a laser focus.
The principle of autonomy: Each note should be autonomous, meaning it should be self-contained and comprehensible on its own. This allows notes to be moved, processed, separated, and concatenated independently of its neighbors. It also ensures that notes remain useful even if the original source of information disappears.