Controlled vocabulary legislate the use of predefined, authorised terms that have been preselected by the designer of the vocabulary, in contrast to natural language vocabularies, where there is no restriction on the vocabulary.
Subject headings are a controlled vocabulary. A single term or phrase is determined by an authority (Library Congress or a database vendor) to describe a subject. For example, try searching for the subject "sources" in West-Cat or WorldCat and your specific movement, and see what you get.
Library of Congress Subject Headings (LCSH)
To retrieve primary sources, use the subject heading "sources" in the subject field box.