If it hits 1k nits, I'll be shocked. That's the only complaint I have about this TV. Not sure what you mean by smaller windows on the TV. Do you mean picture in picture? For some reason, the TV will display different levels of brightness at different times. It's difficult to explain, almost like it's a software bug.
No, window = the tv screen.
So if you have a black tv screen and then you insert a bright object into say, 5% of that entire screen then it's a 5% "window" of that screen, 100% window = the entire screen and so on.
It's basically a way to measure the luminosity that the screen can get by gradually making the area bigger.
With most HDR tv's they are able to make smaller areas brighter, but as you increase the size of the highlighted area it tends to fall off lower and lower, especially once you get to 25% and higher of the screen.
However with the TCL p605/607, the small windows (like 2/5/10% of the screen) the brightness is dimmed and not as bright as 25/50%, which is like backwards compared to most other hdr tv's that tend to be able to make "smaller" windows the brightest that start rolling off once you get past 25% or more of the screen.
What this means basically is that when you are watching a dark scene and then there's a bright highlight, if it's just a small area on the screen it won't be as "bright" as say, a larger object on the screen.