The Chinese CCTV industry appears to use the resolution of the actual sensor, not what the final video output actually is. Which is why these grossly inflated numbers (typical marketing lies) come from. Mostly noticeable with old school analog cameras calming up to 800 lines when the NTSC limit is 480 (more or less as stated).
That’s not what lines in analog video specs mean.
As you can imagine, the number of scan lines is fixed (525 in NTSC, 625 in PAL), with a fair number at the edges lost to overscan (and some mandatory blank lines, etc), resulting in the ~480 visible lines. We are still talking about
scan lines, that is, the lines going from left to right, aka the
vertical resolution.
On the other hand, as
analog standards,
there is no fixed horizontal resolution in NTSC/PAL/SECAM, since it’s a continuous analog signal for each line. So when we say that S-VHS gets around 400 lines, laserdisc 450, broadcast 350ish, and regular VHS only about 250, we are talking about the
horizontal resolution. And this is why DVD looked so good: it does 720 lines. 800 is eminently possible, though it requires exceptionally good cabling and signal processing. Only broadcast monitors could do anything close to that.
As for how it’s calculated, it means that when recording a test chart, you can make out that many alternating black/white lines (hence the starburst test patterns).