In the CIE 1931 model, Y is the luminance, Z is quasi-equal to blue (of CIE RGB), and X is a mix of the three CIE RGB curves chosen to be nonnegative.
CIE 1931 color space - WikipediaLuminance is a photometric measure of the luminous intensity per unit area
(Luminance - Wikipedia)Photometry is a branch of optics that deals with measuring light in terms of its perceived brightness to the human eye. […] Photometry is distinct from radiometry, which is the science of measurement of radiant energy (including light) in terms of absolute power.
(Photometry (optics) - Wikipedia)
However, it’s still a linear quantity (twice the RGB stimulus, twice the intensity). It does represent the relative perceived brightness correctly, though.
Out-of-gamut values need to be tamed before we can perform reliable computations. No tone mapper in darktable handles such colours very well (though filmic is perhaps the most sensitive to them). For an extreme example, see module proposal: gamut compression - #22 by kofa and the follow-up from @hanatos pointing out much of the input (data in the image) is out of the spectral gamut, having been corrupted by the camera input profile.