abap中AT USER-COMMAND.

abap中AT USER-COMMAND.
DATA:NUMBER1 TYPE I VALUE 20,
NUMBER2 TYPE I VALUE 5,
RESULT TYPE I.
START-OF-SELECTION.
WRITE:/ NUMBER1,',NUMBER2.
AT USER-COMMAND.
CASE SY-UCOMM.
WHEN 'ADD'.
RESULT = NUMBER1 + NUMBER2.
WHEN 'SUBT'.
RESULT = NUMBER1 - NUMBER2.
WHEN 'MULT'.
RESULT = NUMBER1 * NUMBER2.
WHEN 'DIVI'.
RESULT = NUMBER1 / NUMBER2.
WHEN OTHERS.
WRITE 'Unknown function code'.
EXIT.
ENDCASE.
WRITE:/ 'Result:',RESULT.
在这段代码中at user-command起到什么作用？