Not exactly what parent poster was saying (I think?), but absorption and scattering coefficients for volume handling together with the Mean Free Path is very wavelength-specific, so using spectral rendering for that (and hair as well, although that's normally handled via special BSDFs) generally models volume scattering more accurately (if you model the properties correctly).
Very helpful for things like skin, and light diffusion through skin with brute-force (i.e. Woodcock tracking) volume light transport.
I might be misunderstanding parts of the comment above, although I think it aligns with what I had in mind. Here’s what I meant:
If a ray carries full spectral information, then a transparent material can be described by its absorption spectrum — similar to how elements absorb specific wavelengths of light, as shown here: https://science.nasa.gov/asset/webb/types-of-spectra-continu...
In that view, transparency is just wavelength-by-wavelength attenuation. Each material applies its own absorption/transmission function to the incoming spectrum. Because this is done pointwise in the spectral domain, the order doesn’t matter:
OUT = IN × T₁ × T₂
(or in a subtractive representation: OUT = IN − ABS₁ − ABS₂).
So whether one material reduces 50% of the red first and another reduces 50% of the green second or vice verse doesn’t change the result. Each wavelength is handled independently, making the operation order-independent.
Very helpful for things like skin, and light diffusion through skin with brute-force (i.e. Woodcock tracking) volume light transport.