subject — One of a group of terms describing the subject matter of a document
Subject terms should be drawn from a controlled vocabulary, such as the Library of Congress Subject Headings. If an outside vocabulary is not appropriate, a local or institutional subject set should be created.
The advantage of a controlled vocabulary is that it places the document into a known subject space. Searching the subject space with a particular subject term will find all of the documents that claim to have that subject. There's no need to worry about terms that are synonymous with the search item, or homophones of the search term.
May be formatted inline or as a displayed block, depending on context. Subjects are rarely displayed to a reader. Usually, they are reserved for searching and retrieval purposes.
Keywords, which may be chosen freely, subject terms should come from a controlled vocabulary.
In order to assure that typographic or other errors are not introduced into the subject terms, they should be compared against the controlled vocabulary by an external process.
Weightspecifies a ranking for this
Subjectrelative to other subjects in the same set.