i was rendering iron man this week. and it got to complicated. i always use ambient occlusion (AO). but if you use AO with raytraced materials then render times get to long. now if you add a complicated mesh plus raytraced shadows and you have problems.
so i decided to render AO as a seperate pass . i would then in photoshop multiply it with the original render. you can not do subtle glows in poser so i also like to render a ''glow'' pass. since i already had so many passes i decided to render almost everything as a seperate pass as an experiment. they do this on movies. they render passes so that they can change everything in real-time.
so the passes are : color,light,AO,matte,specular ,glow