Elemental affinities in this version of the Narutoverse follow a discrete scale, from E- to S-rank (with up to two pluses in between). The higher the rank, the more chakra control and reserves are required. Your natural affinity has to be at least one rank above the attempted technique to accomplish it with half the hand seals required; and three ranks above to accomplish it with a fourth of the hand seals required. Higher ranks also mean that techniques that have equal and/or lesser rank require less chakra to use.