What is the difference with TerrainDetails?

My game instantiates terrain objects and then melds them into one mesh but larger meshes to keep effeciency then adds a collider. Its actually vital to the game and it works but this method feels like it won't work for smaller objects like stones, trees and obviously grass. The thing is that I can add grass and such but how can I get the grass or small objects to load up like the grass and be added in as large quantity?

Whats the bit of the code that causes the difference between objects added as terrain details or trees and the same object placed without the terrain tools? Namely the LOD.