BB, you said, "So if you put it at 90, then the sun will be coming in from the right, no matter how the sIBL was originally oriented".

So 0 will always adjust the sIBL to be directly behind us or toward us? 90 will always be to the right, and 270 would always be from the left?

Also, I'm assuming that's in relation to the default view port as opposed to the current placement of the camera?

Yes - the rotation is identical in value to the yRotate on an infinite light. The reason this is important is that many sIBL load with the sun somewhere at random relative to your scene. When your camera is zeroed, you're pointing "north" and when the light is zeroed, it is coming from the "south", pointing "north". If you want light from the right (east, or 90), as you switch scenes you would have to manually rotate to bring the sun back to the right. By automating this you can avoid having to do that step.

With rotation at 180, the sun is pointing back towards you - making a nice back-lit rim lighting situation. West (left) light is 270.

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