This is based on Pixar's paper 500 million and counting: hair rendering on ratatouille
. First it emits a proxy procedural with a conservative bound, -3 3 -3 3 -3 3 in this case. The proxy procedural will NOT render anything. It emits a number of real procedurals with tighter or exact bound. Those procedurals will emit RIB stream to renderer, a number of spheres with random size in this case. The idea is to subdivide a large date to a set of smaller data, and to delay RIB stream.
Assuming you have 3Delight installed on your system, you can Make it. Open foo.rib to find the line:
Procedural "DynamicLoad" ["spheres" "21"] [-3 3 -3 3 -3 3]
The number follows "spheres" controls how many spheres it will render.