Suppose you have two customization elements extending the same UML element (for example, the same element specified as a customization target in both customization elements). Each customization element has a derived property. If the derived properties’ names, types, and multiplicities are the same, then the expressions defined for these derived properties are merged. Consequently, the extended UML element will have only one derived property instead of two and the property values will be calculated according to the union of expressions defined in both derived properties.
This occurs when the derived property (already specified in, for example, the UML Standard Profile) is also specified in a project. Note that in contrast to the example above the real-life expression merge can be performed in more complex situations with more than two derived properties having more than one expression defined. The expression merge can combine different types of expressions.