Subject cataloguing/indexing involves assigning terms to represent what the document is about. The complete set of index terms in a subject indexing system may be referred to as the vocabulary or index language of that system. In the case of most libraries, documents are arranged on shelves while the index is likely to comprise entries within a card catalogue representing these documents and providing access to them under the selected Index terms to represent their subject matter. In other situations, the index will be in machine-readable form (on magnetic tape or disk), or microfilm, or in printed book form. In most cases the vocabulary used will be a controlled vocabulary i.e., a limited set of terms that must be used by indexers and searchers. The controlled vocabulary is most often used to standardize descriptors or subject headings representing the contents of documents or subject interest profiles of users (i.e., search strategies and users' profile used in SDI). In principle, however, such a tool can be applied in situations in which the standardization of terminology is needed.