There are four kinds of Types: atomic, object, array and union.

Types are either Builtin, in which case their name is in no namespace, or Derived.

The topmost Type is builtin and is named "item".

The topmost Object Type is builtin and is named "object".

The topmost Array Type is builtin and is named "array".

The topmost Atomic Type is builtin and is named "atomic". There are many further Builtin Atomic Types.

Derived Types are always defined by restricting the value space of a base type by means of facets. They have a JSON object representation.

Derived Object Types are always directly derived from "object". Derived Array Types are always directly derived from "array". Derived Union Types are always directly derived from "item". Derived Atomic Types may be derived from any other Atomic Type.