I've never really been sure what that was exactly. All I know is that you need to have it on when making reflections and using metal.

Under what circumstances can it be turned "Off".  I've always had it on no matter what I am rendering, but I'm on an older computer right now and I need to get a handle on more efficient ways of rendering because it doesn't have a whole lot of RAM to work with.

When is Ray Tracing needed, and when isn't it?

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi