LEVEL 66 In COBOL : What is level 66 used for ?

Level-66 Data-item is used for RENAMES Clause. RENAMES clause is used to regroup(Re-arrange), club together existing fields under a new name.

01 RECORDS.
05 GROUP-A.
10 FIELD-1 PIC 9(02).
10 FIELD-2 PIC 9(02).
05 GROUP-B.
10 FIELD-3 PIC 9(02).
10 FIELD-4 PIC 9(02).
66 GROUP-C RENAMES FIELD-2 THRU FIELD-3.