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.