In your case, stationary or non-stationary means time dependence (energy).
Stationary radiative transfer is used as the model in the case of non-interacting particles moving through a medium for example.
The equilibrium distribution of mutually non interacting particles propagating through a scattering
medium is described by the stationary radiative transfer equation
https://arxiv.org/abs/1304.6504
Non-stationary radiative transfer is used as the main model in cases where the particles are considered interacting and the energy levels do change with time (time dependence).
The illumination of the dust nebula
under the influence of radiation of a new star can be considered as an example
of such process.
The simplest model of non-stationary radiative transfer is a model based on
the consideration of the one-dimensional homogeneous infinite medium with an
energy source depending on time.
https://doi.org/10.3103/S106345411701006X