The material is assigned to the volumetric tag and is mostly refracting light using nd1.33.
If you have a cloudy scene roughness 0 works very well, but for sunlight roughness 100 works best in my experience.
Adding some reflectance to the material is ok, but too much makes it behave more like smoke rather than water vapour.
I can only guess at how it works, (no proper documentation is available that I'm aware of
) I think its like your texturing points in space rather than a surface, maybe maxwell samples small spheres rather than points, but I doubt that. I just imagine a single ray refraction+reflection occurs at sampled points, and maybe it also calculates one ray, refracted at an angle that a sphere would have given, had the ray refracted twice (as it exits the sphere).