Pretty much this: https://msdn.microsoft.com/en-us/library/windows/desktop/bb173349%28v=vs.85%29.aspx
I'm almost certain the trees in Outerra are already instanced, and thats why there can be so any of them with so little overhead.
If I'm not wrong, it's already the case: in Outerra each package (geometry - textures) is only loaded one time in the GPU memory. You can have one or 200 instances of the same object, it will have the same memory footprint. However, global objects polygon count still have an impact, I'm not sure if there is a way to change that. One thing Outerra can't do for the moment, is making possible to resize objects.
Edit:
@HiFlyer,
Outerra is already making use of geometry instancing for separate entities. If you spawn the same object multiple times, the instancing will happen automatically.
Try this: Spawn an object, press Alt+7 (you must have debug_keys enabled in eng.cfg) and look at the number of draw calls. Then spawn the same object again and you'll notice that the number of draw calls will stay the same.
Interesting. Is that for the planes, as well. So that a line of parked planes would show the same number of calls as one?
Yep. But don't forget that the sum of each of these objects geometry still have to be computed to render the image. So spawning a lot of complex objects will still affect the framerate, but not the GPU memory.
LFNA has around 1022 objects in view when you're close to the airport, looking at Tallard, but only 240 draw calls.
Talkeetna uses a lot more different buildings so, 513 in view for 353 draw calls.
When you use the alt+7 tool, only the first instance of the object in a yellow box.