Rendering to a viewport rather than to the Render window (default behavior) is accomplished by setting the render destination to the viewport. See “Advanced Rendering” for a detailed description.
Since the regular display drivers in AutoCAD do not have full rendering capabilities for the material properties, rendering to a part of, or to the entire viewport allows you to see the impact of changes to material properties. The rendered image in the viewport is a short term display, like glyphs and highlights, that can be removed by calling acgsRemoveAnonymousGraphics().