please eloborate on the following form code its a small one

hi can anyone explain me followig form

its used in a program of a sap scripta

and when i click on form entry no perform is there for it it says possible dynamic calls

form entry using return_code us_screen.

clear retcode.

xscreen = us_screen.

perform processing using us_screen.

case retcode.

when 0.

return_code = 0.

when 3.

return_code = 3.

when others.

return_code = 1.

endcase.

endform.

regards

arora