By default, all objects cast and receive caustics, but not all have to in order for you to achieve the look you want. You can fine-tune the look of global illumination or caustics (or reduce render times) by specifying exactly which objects should cast and, or receive photons.
Though you can use these per-object (local) settings alone in many cases, usually you use them in conjunction with scene-wide (global) overrides to give you the maximum amount of control over the rendering of global illumination and caustics. (See .)
How you combine per-object global settings depends on what you want to achieve. You can, for example, flag specific objects to receive global illumination, while the rest of your objects in your scene don’t. Or, you can flag all objects to receive caustics, but only one object to generate caustics.
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