Content Designator 

Content designators are used to denote the identity of each data field (instead of identifying data fields by position) within a record. It may be grouped together at the beginning of the record with 'pointers' to the data they identify. The storage of the computer file invariably uses the method of indexing the location of the files at the beginning of the file. The computer calculates from the given starting character position of each file the likely position of that file on the tape/disk, moves the same very quickly to a position just before the start of that file at which point the tape / disk moves more slowly in order to locate the data. This index is known as a 'Directory'.