Hierarchical Level of Detail Overview
Hierarchical LOD Outliner Reference
The Hierarchical Level of Detail System (HLOD) must be enabled on each Level that wants to make use of it. Follow the steps below to enable the Hierarchical LOD System.
Enable by going to World Settings -> LODSystem -> Enable Hierarchical LODSystem
Once this is enabled you can go to the Main Menu -> Window -> Hierarchical LOD Outliner to set up how HLODs are generated for your scene.