In the Inspector window, switch to the third tab, open the Library foldout, this is where you create and manage your environmental objects collection:
To add a prefab to the library, click on the Add button, then select the prefab you want to add. In some case, if the preview image cannot be loaded, an asset re-import is required. But don't worry, this process happens automatically and asynchronously, you have to do nothing!
To remove a prefab from the collection, select the prefab and hit Remove, then OK. Caution: this action cannot be undone at the moment, and all prefab instances, including the grouped one, will be flush out of the terrain.
Each prefab in the collection will be displayed as a tile, with its name shown below. The blue border indicates the currently selected prefab.
In each tile, the number at bottom-right corner indicate how many instances has been spawned to the scene. When click on the three-dot button at the top-right corner will show a context menu with additional actions:
Group: group nearby instances into one larger object, size of the group depends on the Combination Physical Size property.
UnGroup: un-group all instances.
Show All: show all instances of a prefab.
Hide All: hide all instances of a prefab.
Export Distribution Map: Create a grayscale map base on instances position.
Mass Placement: randomly spawn object using the distribution map and other parameters.
Clear All: Destroy all prefab instances as well as its groups.