The SORT statement above sorted all the records in the file into a new file. But if you wanted to produce a sorted file that only contained, for example, product numbers which begin with a '1', you would use an INPUT PROCEDURE.

The record FD might be:

   FD UNSORTED-FILE.
   01 UNSORTED-RECORD.
        03 1ST-DIGIT-OF-CODE  PIC 9.
        03                    PIC X(20).

The description gives the minimum detail required. Now some procedure division:

   PROCEDURE DIVISION.
   SORT-SELECT.
        SORT WORK-FILE
            ON DESCENDING KEY PRODUCT-NO
            INPUT PROCEDURE SELECT-PROD-CODE
            GIVING SORTED-CODES-FILE
        STOP RUN.

So the paragraph SELECT-PROD-CODE might be like this:

     SELECT-PROD-CODE.
         OPEN INPUT UNSORTED-DATA-FILE
         PERFORM UNTIL END-OF-FILE
           READ UNSORTED-DATA-FILE
             AT END MOVE 'Y' TO EOF-FLAG
             NOT AT END
               IF 1ST-DIGIT-OF-CODE = 1 THEN
                 MOVE UNSORTED-RECORD TO WORK-REC
                 RELEASE WORK-REC
               END-IF
           END-READ
         END-PERFORM
         CLOSE UNSORTED-DATA-FILE

If you just want to print specific sorted fields you would use an OUTPUT PROCEDURE. Based on the above example:

   PROCEDURE DIVISION.
   PRINT-SORT-REC.
        SORT WORK-FILE
            ON DESCENDING KEY PRODUCT-NO
            USING UNSORTED-RECORD
            OUTPUT PROCEDURE PRINT-SELECT-PROD-CODE
        STOP RUN.

So the paragraph SELECT-PROD-CODE might be like this:

     SELECT-PROD-CODE.
         OPEN OUTPUT PRINT-FILE
         PERFORM UNTIL END-OF-FILE
           RETURN UNSORTED-DATA-FILE
             AT END MOVE 'Y' TO EOF-FLAG
             NOT AT END
               {move fields in SD sort group to print fields}...
               WRITE PRINT-RECORD FROM {print group}
           END-RETURN
         END-PERFORM
         CLOSE PRINT-FILE.

You can combine INPUT and OUTPUT procedures into the same sort statement by replacing both the USING and GIVING statements:

       SORT WORK-FILE
          ON DESCENDING KEY PRODUCT-NO
          INPUT PROCEDURE SELECT-PROD-CODE
          OUTPUT PROCEDURE PRINT-SELECT-PROD-CODE
       STOP RUN.