Processing
Screens 
There are two ways of calling a screen. You can either use a transaction code, or the CALL SCREEN statement in an ABAP program. When you call a screen, the PROCESS BEFORE OUTPUTevent (PBO) is called, and the corresponding event block in the screen flow logic is processed. The screen itself is then displayed until the user triggers the PROCESS AFTER INPUT(PAI) event by choosing a function. In between, there may be field or input help processing. The corresponding event blocks in the flow logic are processed, if they exist. The main purpose of event blocks in the screen flow logic is to call dialog modules in the ABAP program and to provide the ABAP program with data.
Field Help, Input Help, and Dropdown Boxes