SchemaType is an interface implemented by all schema types: simple and complex types, built-in and user-defined types.
There is a hierarchy of interfaces that extend SchemaType, representing the top levels of the schema type system: SimpleType and ComplexType, with SimpleType further subdivided into List, Union, and Atomic types.
The implementations of these interfaces are organized into a different hierarchy: on the one side, built-in types such as AnyType, AnySimpleType, and the built-in atomic types and list types; on the other side, user-defined types defined in a schema.