The refinement value, specified for each node by the user, is used during the surface mesh generation phase to determine the mesh element size in the neighborhood of the node. Small elements (and more resolution) can be requested at nodes where the solution is likely to be more complex.
The RNG (Reference Node Group) value is used by the simulation software to keep track of which boundary conditions must be applied on each face. For instance, all the faces that together make up the free surface will have the same RNG number. Faces on the river bed will have a different RNG number, and so on.