Max Reflection Photons
Use this to limit the number of times a photon will reflect in a scene (after the first bounce, which is taken care of by direct illumination). It works in conjuction with Max Photon Depth.
Max Refraction Photons
Use this to limit the number of times a photon will refract in a scene (after the first bounce, which is taken care of by direct illumination). It works in conjuction with Max Photon Depth.
Max Photon Depth
Use this to limit the number of times a photon will bounce around (reflect or refract) after the first bounce (which is taken care of by direct illumination) in a scene.
Default is 5, but correct value depends on how many surfaces the photon must go through or bounce off of before hitting a diffuse surface to stop. For example, if a photon goes through 6 transparent surfaces, the default 5 would produce incorrect results. After the depth trace limit has been met, photons are not reemitted and instead are absorbed.
Custom shaders may override these values.