The Zettelkasten is a bottom-up note-taking system
The Zettelkasten is a bottom-up note-taking system, meaning there is little structure imposed to organize or categorize main notes. The structure comes from the relationship between the main notes and the connections they form.
Rather than structuring his zettelkasten around topical categories or as a hierarchical tree structure, Luhmann chose instead to allow his compartment of main notes to develop bottom-up, its structure emerging out of the relationships between ideas, sections emerging over time. [1, p. 105]