Your literature and permanent notes are stored in a specific sequence, with cross-references to other notes. Luhmann used an alphanumeric indexing system.
For example, an existing note may be labelled #10, and a related note will be filed behind it as #10a. The next unrelated idea is labeled 11. By using numbers and alphabets (instead of narrow topics), he could create unlimited branches (e.g., 13/3d8a6 followed by 13/3d8a7) and add the same note to multiple contexts.