Reflective GI caustics
Reflective GI caustics – Allows indirect light to be reflected from specular objects (mirrors, etc). Note that this is not the same as Caustics, which represent direct light going through specular surfaces. This is off by default, because reflective GI caustics usually contribute little to the final illumination, while often they produce undesired subtle noise.
(Note that "reflective GI caustics" is actually turned on by default in V-Ray now, despite the documentation claiming otherwise.) This topic is primarily targeted at V-Ray users but other renderers also support this feature which, however, may or may not allow to turn it off.
So, how does this actually look like?
Reflective GI caustics comparison
You can see that the image with reflective GI caustics turned on is overall a bit warmer and even has a little golden caustics effect on the floor beside the cube. This is caused by GI rays bouncing off of the reflective surfaces and thus, for example, carrying the golden color onto the sphere.
The effect is rather subtle but it adds a little bit more realism to the scene. It should be noted, however, that this is only possible due to secondary ray clamping. If turned off or set too high, we would end up with a very noticeable amount of noise.
When it can be a problem
Reflective GI caustics comparison in an interior scene
In some cases, a combination of bright light sources and very reflective glossy objects can lead to an increased amount of noise. In the image shown above, we can see some additional coarse noise where the right wall meets the floor. Most of the time it is less apparent and thus not as much of a problem. It can often be solved by higher sampling settings but the severeness of the issue is very much dependent on the scene, and other factors like the noticeable increase in render time, especially in animations, may make it worth to have it turned off by default.
It's important to note that this is very different between renderers and versions. Older V-Ray versions, for example, were a lot more prone to excessive noise than newer ones and you should definitely do some tests in your specific case to get an idea of how well your particular renderer can handle it.