Linked from
The 11 pages that link to Radiative transfer equation, each with the reason it gives.
Optical depthNarrower topic: Optical depth is a path coordinate that simplifies this equation.
Integral equationRelated: Scattering couples intensities across directions through integral terms.
RadianceRelated: It tracks radiance as absorption, emission, and scattering alter a beam.
Spectral radianceRelated: It tracks spectral radiance as sources and interactions alter a beam.