$$\require{cancel}$$
This is the intensity emitted per unit volume of a gas. In this definition, the word “intensity” (without being preceded by the word “specific”) is intended to mean what is meant in section 1.4. The symbol used for emission coefficient is $$j$$, and the SI units are $$\text{W sr}^{-1} \ \text{m}^{-3}$$.
The mass emission coefficient $$j_m$$ is the intensity per unit mass, and is related to the volume emission coefficient by $$j = \rho j_m$$. Not all authors are equally clear as to whether they are referring to a volume or a mass emission coefficient.