Refractive GI caustics – Allows indirect lighting to pass through transparent objects (glass etc). Note that this is not the same as Caustics, which represent direct light going through transparent objects. You need refractive GI caustics to get skylight through windows, for example.
Reflective GI caustics – Allows indirect light to be reflected from specular objects (mirrors etc). Note that this is not the same as Caustics, which represent direct light going through specular surfaces. This option is disabled by default because reflective GI caustics usually contribute little to the final illumination, while often they produce undesirable subtle noise.
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When the render engine is set to CUDA, the Reflective/Refractive Caustics options are not available.
Primary Engine – Specifies the method to be used for primary diffuse bounces.