It is normally necessary to check user input for validity and consistency. There are three kinds of input checks on screens:
Automatic input checks are called in the PAI event before data is transported back to the ABAP program and before dialog modules are called.
Input checks in the flow logic can be performed before you call dialog modules.
These input checks are programmed in PAI modules. If the user enters an incorrect value, you can make input fields ready for input again without repeating the PBO processing.