Each derived property must have one or more expressions defining the criteria for searching the result elements.
To open the dialog for a new expression definition
It is strongly recommended that you specify the customization target in the customization element before defining the expression for its derived property.
Select the attribute with the stereotype «derivedPropertySpecification».
Open the attribute’s Specification window.
In the general specification pane (opened by default), click the Expression property and then click the ... button. The Expression dialog opens, and you can then construct predefined expressions or define your custom operation for calculating derived property values.
Select the Expert mode to see the full list of available operations.