Because a REDEFINE statement defines the same section of memory in multiple ways, redefine fields and groups must be the same length as each other. If the copybook does not define these fields to be the same length, the createdatamaps utility inserts FILLER fields in the appropriate locations.
An exception to this rule is a REDEFINE statement that is the last item in a copybook. In this case, the redefine fields or groups can be different lengths because they typically describe different data record types, which can be different lengths.