You can create a primary identifier when you create attributes for a class.
- Double-click a class in the diagram to display its property sheet, and then click the Attributes tab.
- Double-click an attribute in the list to display its property sheet, and then click the Detail tab.
- Select the Primary Identifier check box and click OK to return to the Attributes tab of the class property sheet.
- Click the Identifiers tab to view the new identifier in the list.
- Click OK.