RenderAll is an option for Graphics3D which specifies whether or not PostScript should be generated for all polygons.
When RenderAll->False, PostScript will be generated only for those polygons or parts of polygons which are visible in the final picture.
If RenderAll->True, PostScript is generated for all polygons. The PostScript for polygons that are further back is given before the PostScript for those in front. If the PostScript is displayed incrementally, you can see the object being drawn from the back.
Setting RenderAll->False will usually lead to a smaller amount of PostScript code, but may take longer to run.
There may be slight differences in the images obtained with different settings for RenderAll, primarily as a result of different numerical roundoff in the PostScript code, and the rendering system.
See The Mathematica Book: Section 2.9.14.
See also: PolygonIntersections.