GraphQLNullable is not a simple nullability type but rather an enum that provides three cases: some, none, and null. It's used to represent the presence of a value, the implicit absence of a value (nil), and an explicitly null value, respectively. The main difference between nil and null lies in their representation in generated response models; nil is represented as a Swift optional, while null is represented by the GraphQLNullable enum. This distinction becomes important when working with input types, where explicit null values could represent deletion of a field, whereas implicit nil values might imply no change to the field. The GraphQLNullable type was introduced to make it clear that there's a difference between nil and null and forces users to select a case that signals their intent without question.