Modify Screen

What is PBO event.

Process B4 Output.
This event is triggered before sending the slecetion screen elements to the presentation layer.
In lay man terms before displaying the slecetion screen.

AT SELECTION-SCREEN OUTPUT event is triggered during PBO.

What is PAI.

Process After Input.
This event is triggered after giving inputs to the selection screen.

AT SELECTION-SCREEN , AT SELECTION-SCREEN ON events are triggered during PAI.

Modify screen makes sence only at PBO.
Changes made to screen during PAI have no effect.