ABAP Subroutine basics in SAP

PERFORM read_data USING d_carrid1.
	 			
FORM read_data USING local_carrid.
	
* local_carrid points to d_carrid. When you change local_carrid you also change d_carrid1.
ENDFORM.

Note: When passing by reference the statements USING/CHANGING do not alter the functionality, you can change values with either statement. Just makes it easier to read if you use the appropriate statement.

PERFORM read_data Changing d_carrid1.
FORM read_data CHANGING VALUE(local_carrid)

*Using the VALUE command creates a local variable called 'local_carrid' which contains the value passed to it. This can then be changed within the local subroutine with no effect on the passing variable(d_carrid). Once the program reaches the ENDFORM statement the passing variable (d_carrid) is updated). If the exit command is execurted before the ENDFORM the value is not updated(must reach ENDFORM).

ENDFORM.
FORM read_data USING VALUE(local_carrid).

If you use the USING command instead of CHANGING the global variable will not be updated even at the end of the sub routine(ENDFORM).

PERFORM sub_routine TABLES itab. 	"Itab must be a standard  table
FORM TABLES p_itab STRUCTURE ekko.
ENDFORM.

*note: You can pass tables without using the TABLES statement. Simply use USING/CHANGING.