Data Transport between screens and ABAP programs is automatically handled by the system.
The First data transport occurs after all the PBO Modules have been executed and prior to a screen being displayed. The system will transport the contents of the ABAP work area in the program into the corresponding screen fields.
The Second data transport trigger is done once all the data entry is completed by the user and before the first PAI Module is executed. The system will copy the contents of the screen fields back to the corresponding ABAP work area in the program.