Not quite, you are thinking Lab I am thinking linear RGB. So far I have tried to keep my contribution simple, and the parallelopipeds approximation for gamut achieve that in a raw conversion context, albeit at the expense of nuance. They are easy to visualize and make it intuitive to understand that if a tone falls outside of the relative one for the output color space, it is necessarily out of its gamut. Change the output color space, change the calculus.
In the linear sRGB example above there are a number of options on how to squeeze that tone into the relative raw-converted file, all suboptimal non-linear compromises. At its simplest we could for instance simply clip R and G to 1 and block B to zero. Or we could get more sophisticated than that. I am not a programmer and did not read the code but my understanding is that the checkbox determines what option is kept open early on in the conversion path.
Thank you Jack for your explanation. Some of the points mentioned are difficult for me to understand, as I’m not a colour scientist, but just a hobby photographer interested in colour management. However, I think I got the point. It is not necessary that all 3 channels (R, G, and B) get a value higher than 1 at once, to become influenced by the check box. Even a colour which consists of the components R=1.2 G=0.8 B=0.5, in a certain working colour space, would be clipped to R=1.0 G=0.8 B=0.5 if the check box is activated.
By the way, do I assume correctly that the raw RGB values from the AD-converter first are multiplied by a certain value (for example Rx1.4, Gx1, Bx1.8) to get the white balance and then are multiplied with the Forward Matrix of the DCP-Profile to get XYZ values? At last, if no LUTs are applied by the DCP-Profile, the XYZ values become transformed into the RGB values of the working colour space. Am I right?