COPY a tree 500 times into your scene and you will have 500 individual trees in your scene, which may make your computer ram will most likely start smoking, and your scenefile size could become massive, making it take forever to load back up, or not at all, getting the  "Bryce out of memory" error (if you can even save the scenefile in the first place)

INSTANCE 5,000 trees into your scene, and Bryce will "see" only ONE tree, but not only remembers the placement of the other 4,999, it will render as if they are actually there. So when you save your scenefile, there is actually only ONE tree in it.

That's kinda oversimplification, but that's the basic idea.

Of course scenefile sizes will increase somewhat when using Instancing, because of Bryce needing to "write down" where all those Instanced objects are, but it will be nothing in size compared to saving a scene with a ton of regular copied objects.

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