I'm almost always talking about the "Real Scene" values, the top figure in the rtings brightness section. Rtings only recorded a value of 725 nits for the 25% window on the X900H, the 10% figure is 450 and Real Scene is 500. Generally when you see the leaked brightness readings from chinese, etc websites they will be quoting the 10% figure.
The 25% window is important too for large bright areas like an interior shot of a big bright window, but 2/10% and Real Scene is better for knowing how good general HDR impact will be.
2/10% is for the brightness of small highlights, like the sun or headlights, since those things will generally not take up a large area of the screen, nowhere near 25% certainly, even a 10% window will be bigger that them most of the time so 2% is best for measuring that.
Real Scene is a way to see how bright 2, 5 and 10% windows will be in actual mixed content and not just a white box over a black screen. This is the video Rtings use (Its the SDR version, but its the same video) for the Real Scene test:
If you have a look at the shot, (this is rough percentages ofc, but you get the idea) the striplights at the top act as the 2% window, the smaller white square on the left is the 5% window and the white square on the right is the 10% window. They made this test so you can see how bright those sized highlights will really be, as opposed to just what you read from a test pattern.
Here is the comparison for X900F vs. X900H I checked.
Compare the Sony X900F and Sony X900H TVs
www.rtings.com
I usually look at the Real Scene + 2% + 10% + 25% values and then compare and contrast them. The Real Scene and 10% should be similar unless they are fucking around with the test patterns* and hopefully the 2% should be the same or just a bit under those first two values, though sometimes its much less if the TV has large dimming zones. The 25% and 50% figures are good for seeing how much brighter very bright scenes would be on an LCD vs. OLED, like the Matrix scene in the white construct area, with Morpheus. Maybe also for really bright games, like the Mount Volbono level in Mario Odyssey.
*Occasionally the 10% figure is much more than the Real Scene figure but thats usually due to a power limit, ie when the rest of the screen is pure black it has the power available to maximise the light output of the 10% window, but when the rest of the screen has content on it, it doesn't have the power leftover to keep a 10% of the screen at the same brightness. It only really applies to super high end LCD sets though.
Sorry for the lecture if you already knew that but while explaining why you shouldn't say X900H is 700 nits, I thought I'd also write out a guide for helping people understand the rtings brightness measurements a bit more.