Ummm, it is described in manual. "The BSP algorithm is most efficient when rendering scenes that include clustered or overlapping objects." So Bryce doesn't render every facet of every object, but just those things that are visible. Ditto for optimisation: "Aggressive This is a more extreme version of Minimal mode. It is best used for complex scenes with localized concentrations of objects. If you have imported DXF objects in your scene, this mode will speed up rendering considerably." And yeah, so there are advantages, obviously. In render times. :-)

-- erlik