The clipping indicators for shadows and highlights have something called a threshold. In my case, default I believe, it is 8 for shadows and 253 for highlights. In 8 bit color space I take that to mean that values less than 8 or greater than 253 will indicate clipping (Note: according to Rawpedia shadows and highlights are treated differently with respect to how many channels have such values) . However, when editing raw files the color depth is supposed to be greater than 8 bits. How much greater varies, I think, depending on the camera. Also, image files can be created in standard formats with 16 bits of color depth. This suggests to me that the thresholds cannot be absolute.
Might it be that these should be interpreted as the values equivalent to 8 and 253 when the color depth is different than 8 bits?