Customizing the metamodel : Metamodel graphical editor : Adding a user definition chapter to a user definition type
  
Adding a user definition chapter to a user definition type
Now you can add these properties found in the User Definition Chapter and placed them into the User Definition Type.
1 From the Explorer, right-click the Definitions group and select the User Definition Type that you created. Double-click the definition to open it or select the definition and right-click and select Open.
2 Under the Chapters tab, click Choices. Drag the User Definition Chapters that you created into the Chapters property.
User Definition Type dialog
3 Click OK.
See also
Creating user symbol types