While you are creating an entity constraint with the Entity Constraint Wizard, use the Attributes page to select the attributes that will be participating in the key constraint definition.
Available
This is the list of the object's attributes that can participate in the key constraint. Click an attribute name and click the right arrow to move it to the Selected list. You can select more than one item by control-clicking. To move all of the available attributes to the Selected list, click the double right-arrow.
Selected
The attributes in the Selected list will participate in the key constraint. To remove an attribute name, click its name, and then click the single left arrow. You can select more than one item by control-clicking. To remove all of the available attributes, click the double left arrow.