Thanks a lot @BananableOffense for the insights !
Indeed, I should have clarified it… I’m already looking at linear RGB and tonemapping is turned off. I’ve also set the exposure accordingly (see Auto Exposure in Unreal Engine | Unreal Engine 5.5 Documentation | Epic Developer Community) so I’m sure I’m getting luminance (in cd/m2) as an output, which I was able to also validate with a simple analytic set up 
My questions are more regarding the physical meaning of the channels (like, can they be interpreted as the sum of the light energy over given wavelengths and, if so, what are the band of wavelengths).