Writeback expressions can be used as a way to allow users to manually edit a value calculated by the expression and write that change back to the database by editing one of the input valuetypes. See one example below for how configure such an expression that calculates hours worked as FTE * normal work time and allows the user to enter hours worked to edit FTE which is the valuetype.
An app using the dataset with this ValueType Expression could look like:
where both rows marked with red here can be edited but all changes on both changes the other one AND writes the resulting change back to the valuetype PersonnelFTE.