COBOL 66 Level
The 66 level assigns an alternate name to a field or group. It doesn’t add a new field to the record, it just assigns an alternate name to an existing field.
You must use the level number 66 for data description entries that contain the RENAMES clause. A level-66 entry cannot rename another level-66 entry, nor can it rename a level-01, level-77, or level-88 entry.
->->--66 data-name-1 RENAMES data-name-2--*----------------*---><- *--THROUGH--*--data-name-3--* *--THRU-----*
So, the key point is it does not take any extra space. it is just like logical re-naming. This renaming does not take additional space. And, your copy book layout also does not increase.
Mainframe languages, especially COBOL, often reuse, or “redefine” an area in a record to save space.
01 SUBSCRIBERS. 05 TYPE-OF-NAME PIC X. 05 INDIVIDUAL-NAME. 10 LAST-NAME PIC X(12). 10 FIRST-NAME PIC X(8). 05 COMPANY-NAME REDEFINES INDIVIDUAL-NAME PIC X(20). 05 ADDRESS PIC X(20) 05 CITY PIC X(15) 05 STATE PIC X(2) 05 ZIP PIC X(5)
By redefining, you can save space. Then the question is how it differs from Level -66. Yes, Redefines save space.
Suppose, in a COPY book, you want to use four 01 level of each 1000 bytes. Then, you need to declare ‘1000’ bytes 4 times. This you can use redefines to avoid multiple time declaration. So, you are saving space here.