I needed to call the modified method on a form's datasource's field to invoke the logic, which updated a couple of fields on the form. I had defaulted a value in the field on the table's initValue()-method but this - of course - means that the modified method is not in play.
Therefore I needed to call the method in code, for which I had two options, either calling the form controls method by setting AutoDeclaration to Yes and then calling modified or calling the datasource table's field's method which I chose.
e.g. ProdTable_ds.object(fieldnum(ProdTable, ItemId)).modified();
but where to place the code? After some time I found out that if I placed it on active() of the datasource the form showed the correct information.