Terragen TGO Format
The Terragen TGO format is Terragen's native object format. TGO stands for Terragen Object. TGO files have a ".tgo" file extension.
The TGO format can save an object and all its attached nodes defining materials etc. If you import an object in another format and then make changes to the materials which you want to keep you could save the object as a TGO and all those changes will be preserved.
To save an object as TGO you should context click on its node in the network view. All objects than can be saved/exported will have a "Save Object File…" item in their context menu. Choose the Terragen Object Format from the Format popup in the Save dialog.
Not all objects can be saved. In fact at this time it's only objects originally in TGO, OBJ or LWO formats that can be saved. Some TGO objects cannot be saved because the object's creator doesn't allow it. It also isn't possible to save TG's built in objects such as Grass clumps, Rocks or Spheres. These objects can be saved as part of a Clip file though.
Context clicking shows a contextual menu. On Windows this would normally be done with a right click. On OS X this could be a control-click or a right click.