SY-UCOMM vs. OK_CODE in Dialog Program
SY-UCOMM is for doing the functions what the user wishes to do at that particular event. You use it in menus and other place. this mainly in using.
OK_CODE is generally used in screen as of I have used. You will define the function in the screen and you can use it in the main program.
OK_CODE acts just as a temporary variable that stores the value of SY-UCOMM.
When user interacts with the screen elements, the function code that you have assigned is filled in the sy-ucomm field which is turn gets reflected in OK_CODE.
In your ABAP programs, you should work with the OK_CODE field instead of SY-UCOMM.
There are two reasons for this: Firstly, the ABAP program has full control over fields declared within it, and secondly, you should never change the value of an ABAP system field. However, you should also always initialize the OK_CODE field in an ABAP program for the following reason:
In the same way that the OK_CODE field in the ABAP program and the system field SY-UCOMM receive the contents of the corresponding screen fields in the PAI event, their contents are also assigned to the OK_CODE screen field and system field SYST-UCOMM in the PBO event.
Therefore, you must clear the OK_CODE field in the ABAP program to ensure that the function code of a screen is not already filled in the PBO event with an unwanted value. This is particularly important when the next PAI event can be triggered with an empty function code (for example, using ENTER). Empty function codes do not affect SY-UCOMM or the OK_CODE field, and consequently, the old field contents are transported.