I agree, it would be useful if Apple could be clearer by what they mean by a derivative. I recall reading somewhere that it's a reduced resolution, grayscale co

I agree, it would be useful if Apple could be clearer by what they mean by a derivative. I recall reading somewhere that it's a reduced resolution, grayscale copy of the image. I can't vouch for that, but that would be a plausible notion of what the "derivative" would be. Personally I would also be placing a hard watermark in the middle of the image, or maybe some hard slashes randomly through the image, so that "clean" images cannot leak out of human review. Let's imagine that the derivative is a 0.5 megapixel, grayscale, watermarked, HEIC-compressed copy of the original image. This would be plenty to determine with zero ambiguity that the image is actually "A1" classified, i.e. depicts a prepubescent minor ("A") engaged in a sex act ("1").