AT SELECTION-SCREEN OUTPUT.
LOOP AT SCREEN.
IF p_radio1 = 'X'.
IF screen-name = 'P_FIELD2'.
screen-active = '0'. "Disable the field
MODIFY SCREEN.
ENDIF.
ENDIF.
ENDLOOP.

AT SELECTION-SCREEN

Purpose:
This event is triggered after the user has entered data and pressed Enter or Execute.

Use Case:
Used for input validation and custom checks on user input before the report proceeds.

Example:

AT SELECTION-SCREEN.
IF p_date < sy-datum.
MESSAGE 'Date cannot be in the past' TYPE 'E'.
ENDIF.