Ali, its not a matter of using realflow vs afterburn. Realflow is a fluids & dynamics simulation engine. It calculates particles based on dynamics a lot better and more realisticly than max can without heavy scripting or additional plugins (glu, rf). Inter-particle collision based on volumetric particle sizes, masses, air pressure, submersive pressure and all that. It is a very handy tool, but what you would use it for is to calculate the dynamics for the scene your are using, and then incorporate that into 3dsmax and then use afterburn on the precalculated particle simulation. okee?
As for afterburn itself, it is not generally used for fire. There are other plugins that handle fire a lot easier than afterburn can, such as phoenix and aura. Afterburn is volumetric and almost fluid based, so therefore it can handle everything that comes under those two categories, water, oil, smoke, dust, explosions, clouds, rocky outcrops, displacement (sortof) e.t.c. Afterburn works off particles, therefore most things that you would think to use with particles, you can use afterburn to help achieve a realistic look. What I have found is that it does actually reduce your render times to use afterburn. It creates the "puffs" for each particle which you then use the noise (or custom maps) to break up, model, conform, colour, light and display each particle. So in essence you can create what looks likese hundreds of thousands of particles with just one. Understand? Think of it as using 3d facing mapped particles. Cubes, Cylinders, Spheres, Metaballs and then adding density falloff and then customising its shape and texture, brightness and colouration. All very fun and I definately do recommend it. But not for fire.