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REDEFINES lets two data items occupy the SAME memory. Useful for parsing fixed-width records that have different shapes.

       01 RECORD-IN.
           05 REC-TYPE     PIC X.
           05 REC-DATA     PIC X(100).
           05 PERSON-DATA  REDEFINES REC-DATA.
               10 P-NAME   PIC X(30).
               10 P-AGE    PIC 9(3).
               10 P-FILLER PIC X(67).
           05 ORDER-DATA   REDEFINES REC-DATA.
               10 O-ID     PIC X(10).
               10 O-AMOUNT PIC 9(7)V99.
               10 O-FILLER PIC X(81).

The same 100 bytes (REC-DATA) are interpreted three ways:

• As raw text
• As a person record (name + age)
• As an order record (ID + amount)

You pick the interpretation based on REC-TYPE:

       PROCEDURE DIVISION.
           READ INPUT-FILE INTO RECORD-IN
           IF REC-TYPE = "P" THEN
               DISPLAY "Person: ", P-NAME
           ELSE IF REC-TYPE = "O" THEN
               DISPLAY "Order: ", O-ID
           END-IF.

Equivalent to a C union — but more declarative.

COMP / COMP-3 / BINARY — different storage formats:

• PIC 9(5) — display (1 byte per digit, 5 bytes total)
• PIC 9(5) COMP — binary (4 bytes for an int)
• PIC 9(5) COMP-3 — packed decimal (3 bytes; 2 digits per byte + sign nibble)

Mainframe data files often use COMP-3 to save space. REDEFINES + COMP-3 lets you parse them.

Implicit decimal point with V:

       01 PRICE   PIC 9(5)V99 COMP-3.

Stores 7 digits with 2 implied decimals — e.g., the value 12345.67 is stored as bytes 12 34 56 7+.

PIC editing characters for output formatting:

       01 DISPLAY-PRICE  PIC $$$,$$9.99.
           MOVE PRICE TO DISPLAY-PRICE.
           DISPLAY DISPLAY-PRICE.   *> $1,234.56

• Z — zero suppress
• * — fill with asterisks
• $ — currency
• , . — separators
• + - — sign

This is COBOL's killer reporting feature — built-in formatting templates that financial reports rely on.