In a physical (pen and paper) Zettelkasten which is following how Luhmann did it, new notes are placed behind or close to existing notes that are most similar or most related to the new note.
This means that one has to think about and find what existing notes of such character are present in the Zettelkasten already, as a necessary step, otherwise the new note will not have an address/location.
This act of being forced to look for similar ideas to new ones helps do two things:
Digital variations on the Zettelkasten method, like what I use, should enforce linking to similar notes, in order to be effective.