In a new database, selecting the sum field I see in the data property Function, Table field, Sum, and context group. In the example I uploaded, I just see a User Function and it's name. In effect it works, but when opening a report for maintenance, the end user does not understand clearly what is running on the field. Finally, if you play with the dialog, you see that the functions are nested and the end-user is not able to manage the definition.
In the first example, a report showing the data section correctly.
In the second, a nested data section.
In my opinion the exact behaviour odb has to use is the first.
Franco.