int32 NumIndirectLightingBounces
Number of light bounces to simulate for point / spot / directional lights, starting from the light source. 0 is direct lighting only, 1 is one bounce, etc. Bounce 1 takes the most time to calculate and contributes the most to visual quality, followed by bounce 2. Successive bounces don't really affect build times, but have a much lower visual impact, unless the material diffuse colors are close to 1.