This section describes how to add an association of type reference set to your domain model. The related reference guide article can be found here.
Create the association between the two entities, choosing parent and child as you wish. If you do not know how to do this, refer to this article.
Select the association.
In the Properties window, use the drop down menu next to ‘Type’, and choose ‘Reference Set’.
The association is now changed to be a reference set, instead of a reference.
The above screenshots show the same association; the first shows it using hierarchical notation, whereas the second shows it using UML notation, which was added in version 2.5.1. You can switch between these modes using the Edit > Preferences menu.