After some further research, i see that the clipping overlay (red) happens when any one channel hits the 255 value (and the triangle indicator in the historgram indicates the channel colour (either R, G or B) or triangle is white when all three hit 255... somewhere in the red overlay area.
At least i know now whats happening, thanks for getting me there.
so for me to see an overlay that is indicating pure white (as i need pure white backgrounds sometimes) i can only rely on the photoshop action i setup to make a layer with a 255 'blend if' property
As this video https://www.youtube.com/watch?v=Iwp6jg8g5xo
Generally, it's more useful to be alerted to gamut clipping rather than highlight or shadow clipping as such.
Gamut clipping happens whenever any one channel hits either 0 or 255. That's a saturation limit in the target color space. Visually, gamut clipping appears as loss of texture and a dense appearance lacking in "air". It doesn't look good.
Presumably, that's what ACR shows (I'm travelling without access to PS/ACR at the moment). Personally I never use these overlays. I'm very much concerned with gamut clipping, but I use the histogram and eyedropper to keep tabs on it.
The point I'm trying to make is that maybe you're using a tool that wasn't really intended for that purpose.