Which of the following elements can't be used to create calculated attributes in an Aggregate?

Calculated attributes in an Aggregate are used to derive new values based on existing data from the Source Entities, which enhances the capability for data manipulation and presentation in application development.

The reason the choice of server actions using entity attributes is not valid for creating calculated attributes lies in the fact that calculated attributes must be evaluated at the query level for efficiency. Server actions are executed at runtime and cannot be directly incorporated into the query execution context of an Aggregate, where calculated attributes are evaluated. Executing server actions would require additional processing after the data has been retrieved, which undermines the purpose of calculated attributes designed to be evaluated in a single database operation.

In contrast, the values of the attributes from source entities directly reference the data available in the database, making them suitable for calculations. Built-in functions translate directly into SQL statements, allowing for efficient computation within the database itself. Variables can hold data that can be used within the context of the application, but they do not directly pertain to the core database query context that defines Aggregates.

Overall, the structure and execution model of the OutSystems platform dictate the appropriate use of each element, confirming that server actions do not fit the framework necessary for calculated attributes within an Aggregate.