Need help with Hierarchical Level of Detail

Hello to everyone!

Can me someone to help to understand in detail the system of LODs.

By the documentation and tutorial shows how to do it through FBX - it more or less clear, but it is not clear how works Hierarchical LOD System (it place in world settings). It works with LOD Group or only allows to minimze the call draws, by merge all meshes and materials-depending on the distances between the objects (it will be merge LOD0 or LOD1?). HLOD need HierarchicalLODVolume or volume can use separately?

Thanks for answers