GI Relative Detail controls the overall detail *relative to the main Detail setting*. So if you adjust the main Detail slider it will also affect the GI results. Think of it like a multiplier for the primary Detail level that is specific to GI (although that's not actually how it works as far as I know). So for example if the main detail is 0.5 and GI Relative Detail is 2, multiply 0.5 by 2 and you get a theoretical GI Detail of "1". If you raise the main Detail slider to 1, your GI detail is now *2*, essentially. It's important then to understand that it is *relative* detail.
GI Sample Quality controls the "accuracy" of individual GI samples in the scene. Increasing this setting will increase the accuracy of lighting and using this in conjunction with a lower GI Relative Detail can produce decent results without as much of a render time increase.
GI Surface Details is an enhancement setting that works on smaller-scale features to improve their lighting quality, but at a significant render time cost. I seldom use this feature.
- Oshyan
