To design a flexible and dynamic application form, in addition to user exits you also need own data areas that you can fill and read when you process forms. You have to declare these data areas as ABAP variables in the user top include.
The user top
included is included in the generated print program of an application form.
The ABAP variables defined there are therefore available in all user
If you use Smart Forms and PDF-based forms, the variables defined in the user top includes are transferred to the generated module of the Smart Form in addition to the complex data type.