Documentation for radiation in Castro

Thanks again for developing WarpX and making it available!

I was looking at the documentation for radiation here:
https://amrex-astro.github.io/Castro/docs/radiation.html
But as an outsider to the astro community, I don't have a clear picture of the radiation transport equation that are being solved, and the documentation does not explicitly show these equations (unlike e.g. the hydro page of the documentation: https://amrex-astro.github.io/Castro/docs/Hydrodynamics.html)

Would you have a reference (e.g. journal paper) that explains the system of equations that is solved with radiation is turned on in Castro?

Start with docs/radiation.html and compare its coverage with docs/Hydrodynamics.html. Identify the radiation transport equations Castro solves and locate an authoritative journal reference; the work is done when the documentation presents the equation system and includes a suitable citation.