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.
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 for the shader or Indirect Specular Blur globally.
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
Specify whether to use the environment color for reflection rays where there was insufficient ray depth (true), or the color specified by reflection_exit_color (false).
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.