So, if using the 'panoramic image' type background, the image can be an HDRI, but it doesn't have to be, is that right?   If it is an HDRI, then how does it behave differently compared to a 'normal' panoramic image?   Is it that it responds better to me adjusting the exposure on a rendered image where it's in the background?   Part of the reason I ask is to understand the pros and cons of using an HDRI panoramic background vs a 'physical sky' background.