It's independent on the code you use. The surface energy is defined by the difference between the slab and bulk divided by the surface area. With this definition, you can calculate the surface energy of any surfaces you want. I think you should read several relevant papers to understand what it is and what it is used for. There are many literature about it. e.g. this PRB paper from Ceder's group Article First-principles study of surface properties of LiFePO4: Sur...