The source function is a characteristic of a stellar atmosphere, and in the case of no scattering of photons, describes the ratio of the emission coefficient to the absorption coefficient. It is a measure of how photons in a light beam are removed and replaced by new photons by the material it passes through.
The source function is a feature of stellar atmospheres, and in scenarios without photon scattering, it represents the ratio between emission and absorption coefficients. It quantifies how photons within light beams are removed and replaced by new photons as they pass through matter.