| Microsoft XML Core Services (MSXML) 5.0 for Microsoft Office - XML Schemas |  |
| Microsoft XML Core Services (MSXML) 5.0 for Microsoft Office - XML Schemas | |
The set of type definitions for a schema form a hierarchical tree with a single root known as the Type Definition Hierarchy. The root of this tree is called a ur-type definition. The ur-type definition is unique because it can be used as a complex type or a simple type, depending on the context of the definition.
Ur-types are unconstrained. They are the base type for all simple and complex types.