In COBOL, 66 level is a special level and every programmer must know. Since this is a special level in COBOL.
What you need to know
COBOL 66 assigns a new name to a field or group. It just assigns an alternative name to a field.
COBOL 66 Level Example
66 dataname_1 RENAMES dataname_2
05 WW-V1 PIC XX.
05 WW-V2 PIC XXX.
10 WW-V3-1 PIC X.
10 WW-V3-2 PIC XX.
10 WW-V3-3 PIC X.
10 WW-V3-4 PIC XX.
05 WW-V4 PIC X(06).
05 WW-V5 PIC X(10).
05 WW-V6 pic x(04).
66 WW-RNM1 RENAMES WW-V2 THRU WW-V3-2.
66 WW-RNM2 RENAMES WW-V2 THRU WW-V3-4.
So physically it does not add any field to copy book. It is just a logical name. You cannot rename 01, 88, 77, 66 level numbers.
You can do RENAMES to regroup the data elements.
Recently my friend has been asked the same question in an interview.