A name that exists within a hierarchy of names can be made unique by specifying one or more higher-level names in the hierarchy. The higher-level names are called qualifiers, and the process by which such names are made unique is called qualification.
Qualification is specified by placing one or more phrases after a user-specified name, with each phrase made up of the word IN or OF followed by a qualifier. (IN and OF are logically equivalent.)
In any hierarchy, the data-name associated with the highest level must be unique if it is referenced, and cannot be qualified.
You must specify enough qualification to make the name unique; however, it is not always necessary to specify all the levels of the hierarchy. For example, if there is more than one file whose records contain the field EMPLOYEE-NO, but only one of the files has a record named MASTER-RECORD:
The rules for qualifying a name are: