The oct-tree stuff is an optimizing gig that Imagine does while ray tracing.
It sub divides the world up into cubic areas - the oct-tree level tells the renderer how much to keep sub dividing things (this is what's going on when Imagine says, "Initializing"). A high oct-tree level works great for most objects, but for very simple objects (like, just a single, simple primative) the renderer over does the sub-dividing. A lower level number would be more effecient for the simpler objects.
The oct-tree area is directly connected to the global world size. Setting the global world size to 0, 0, 0 makes Imagine take it's best guess at a world size for the oct-tree.