This switch in the Standard Surface shader specifies whether specular or transmission bounces behind diffuse bounces are enabled or not. As caustics can be noisy, these are disabled by default.
To control noise from caustics, the global Indirect Specular Blur setting may be increased, which blurs out caustics to reduce noise at the cost of accuracy.
Arnold will require a high number of Diffuse samples to achieve a clean image when using caustics. Care should be taken when enabling this feature.
Reflective caustics (left), refractive caustics (right). Rollover images to view without Caustics.
Notice that the areas in the scenes below are darker inside the glass. This is because the illumination inside the glass is essentially all caustics. Enabling caustics for the glass shader fixes this issue.
Rollover images to view with Caustics enabled
You may experience fireflies when using high Specular Weight, and low Specular Roughness with Caustics enabled. To reduce this type of noise, increase the Specular Roughness of the shader or Indirect Specular Blur globally.
Enabling caustics can add more believability when rendering eyes.
Rollover image to view caustics enabled for outer cornea shader
Unchecking internal reflections will disable indirect specular and mirror perfect reflection computations when ray refraction depth is bigger than zero (when there has been at least one refraction ray traced in the current ray tree).
In the right image below, the sphere appears black because Internal Reflections is disabled in the Standard Surface shader assigned to the sphere.
Exit to Background
This will cause the Standard Surface shader to trace a ray against the background/environment when the maximum GI reflection/refraction depth is met and return the color that is visible in the background/environment in that direction. When the option is disabled, the path is terminated instead and returns black when the maximum depth is reached.
The amount of diffuse light received from indirect sources only.
The amount of specularity received from indirect sources only. Values other than 1.0 will cause the materials to not preserve energy, and global illumination may not converge.