The screen flow logic does not contain any data declarations. With the exception of the OK field that is generated when the screen itself is created, all other data objects of a screen (screen fields) created when the screen elements are defined in the Layout Editor are assigned to the latter. The data types of screen fields are defined either through reference to the built-in data types of the ABAP Dictionary or by reference to global fields of the ABAP program. After PBO processing and before the screen is sent, there is a data transport of global data objects for the respective ABAP program to screen fields of the same name. After a user action on the screen, and before or during PAI editing, the data transport takes place in the reverse order
At PBO time, all screen fields are transported at the end of PBO processing, with the exception of fields that are defined in table controls or in step loops. The latter are processed in loops in the flow logic and transported from the ABAP program to the screen after each loop execution. At the time of PAI, first the content of all screen fields that do not belong to any table control or step loop and are not listed in any FIELD statement are transported into the fields with the same name in the ABAP program. The contents of the fields of a table control or step loop are transported to the ABAP program row by row or group by group at the beginning of the respective loop run. The fields that are listed in the FIELD statements of the screen flow logic are transported upon execution of the corresponding FIELD statement.
If screen fields with reference to flat structures are defined in the ABAP Dictionary, the global data objects of the ABAP program with the same name must have been declared as interface work area using the statement TABLES. Otherwise, no data transport will take place.