Entity reference field that comes with Drupal core allows creating a linear list of entity references. This module allows creating hierarchical entity references where each entity reference is in a parent/child relationship with another entity reference. It also supports revisions if used together with Entity reference revisions module. Example: Reference (delta 0)(depth 0) Reference (1)(1)  Reference (2)(1) Reference (3)(0) Reference (4)(1)  Reference (5)(2)