When an indexer uses controlled vocabulary in building his or her index, he or she must use words to describe documents that have been "pre-approved" in the thesaurus that governs the controlled vocabulary for the index.
For example, the thesaurus for an index may specify that the indexer use the term "African-Americans" over "Blacks."
Consequently, a user of the index may need to consult the thesaurus to determine the appropriate word(s) under which to search.
Of course, each index will have its own thesaurus. A general-use index might do fine with assigning the term "train," whereas a transportation-related index may forced the indexer and searcher to make distinctions between "subway trains," "light rail transit," "monorails," "passenger trains," and "freight trains."
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