From The Verge:
The funny thing about the 12.9-inch iPad is that it is very easy to miss the benefits of Mini LED in normal day-to-day use. At first you don’t see it.
Sure, there’s great contrast when you’re browsing the web, texting, playing games, and so on, but really it’s not very different from any other iPad. Apple still limits the max brightness in most scenarios to 600 nits, which is bright enough but not eye-popping (the iPad and iPad Air max out at 500 nits).
The magic kicks in when you are viewing videos or photos in full-screen. When you do that, the iPad Pro kicks into a different HDR mode (or in Apple’s parlance, XDR, for “Extreme Dynamic Range”) that really is stunning. The overall max brightness of the screen jumps up to a powerful 1,000 nits and peak brightness for certain lighting can hit 1,600 nits.
You don’t see it until you see it — but then you see it.