Backward-linking
The important implicit relationship between nodes in Luhmann is the “follows-after” relation. It allows browsing the Zettelkasten in a (quasi)linear way. (Some people disagree, some disagree on the disagreement.)
The hierarchical “paths” allow nodes to have multiple “follows-after” relations, e.g 2, 1a, 1b, 1c all directly follow 1.
In the Caudex, this relationship is represented using forward and backward links, which are generated automatically by caudex.el.