$$\require{cancel}$$
If the predominating mechanism is scattering with no absorption, we can define in a similar manner linear, atomic and mass scattering coefficients, using the symbol $$\sigma$$ rather than $$\alpha$$. For the physical distinction between absorption and scattering, see Section 5.1. And if both absorption and scattering are important, we can define linear, atomic and mass extinction coefficients, using the symbol $$\kappa$$, where
$\kappa = \alpha + \sigma.$