Nerfs don’t contain a surface representation, but instead contain a occupancy field to represent the shape. This has implications for rendering speed (you generally need more samples along each ray to get an accurate representation). It also has implications for uses outside of rendering, e.g. in physics simulations where you need to know exactly where the surface is. Lastly, you can use SDFs to get arbitrarily detailed meshes by sampling the field at some resolution, who which is trickier with nerfs because the surface is more fuzzy.