Catalogers assign each subject—such as author, topic, series, or corporation—a particular unique identifier or heading term which is then used consistently, uniquely, and unambiguously for all references to that same subject, which obviates variations from different spellings, transliterations, pen names, or aliases. The unique header can guide users to all relevant information including related or collocated subjects. Authority records can be combined into a database and called an authority file, and maintaining and updating these files as well as "logical linkages" to other files within them is the work of librarians and other information catalogers. Accordingly, authority control is an example of controlled vocabulary and of bibliographic control.