Customizing the metamodel : Metamodel graphical editor : Creating user definition types
  
Creating user definition types
The User Definition Types are used to build up the User Diagram Types. For example if you created a Library Diagram you will want to add definition types to support your modeling scenario.
To create a user definition type
1 From the Explorer, right-click the Definitions group and select New Definition, or click Dictionary > New Definition.
2 Select User Definition Type and name it.
3 Click OK.
See also
Creating user definition properties