Represents the root User Dictionary object.
The user dictionary defines a hierarchy of user object type names
used for dynamically classifying objects. These type names are
defined by the user organization according to the needs of
the disciplines represented in the data. The user type of any
object may be changed as the data becomes more precise.
The hierarchy of type names starts with base names, defined by
the system, for each type of object. The user organization may
define or change the hierarchy of type names under each base name.