FORM ENTRY USING RETURN_CODE US_SCREEN

Hi experts,

What does this part of code do in the driver program of each smart form if the output is triggered using some output type.Say If I go to a delivery and issue the output of the delivery the control comes to this part of code.I want to know the reason or rather the flow of control in this code.

FORM ENTRY USING RETURN_CODE US_SCREEN.

CLEAR RETCODE.

XSCREEN = US_SCREEN.

PERFORM PROCESSING USING US_SCREEN.

IF RETCODE NE 0.

RETURN_CODE = 1.

ELSE.

RETURN_CODE = 0.

ENDIF.

ENDFORM. "entry

Any pointers are welcome.

TIA

Abhishek