Caustics - Arnold for Maya User Guide 4 - Arnold Renderer
angels egg

Arnold uses uni-directional path tracing and so 'soft' caustics originating at glossy surfaces are perfectly possible, as well as caustics coming from big sources of indirect light. The caustics switches in the Standard shader mean that you can tell diffuse GI rays to 'see' the mirror reflection, glossy reflection and refraction from the shader of the surfaces that are hit by them.