Requirements 

The important requirements of a controlled vocabulary are as follows: 

1. It should have 'warrant' derived from the terminology of literature and the information needs of the actual or potential users. That is to say, a term is justified (warranted) if (i) literature on this subject is known to exist, and (ii) requests for information on this subject (denoted by the term in question) are likely to be made fairly frequently. 
2. It must be sufficiently specific to allow the conduct of the great majority of searches at an acceptable level of precision. This implies that the level of specificity will vary over the vocabulary, some subject areas being developed in greater detail than others. A 'vocabulary developed by National Library of Medicine would need only a few general terms in Mathematics while one developed by the American Mathematical Society would need only a few general terms of a medical nature. 
3. It should be sufficiently pre-coordinate to avoid most problems of false coordination and incorrect term relationships. One way of achieving this, and also of economizing on the absolute size of the vocabulary, is through the use of subheadings. 
4. It should promote consistency in indexing and searching by the control of synonyms, near synonyms, and quasi-synonyms. 
5. It should reduce terminological ambiguity through the separation of homographs and through the definition of terms whose meaning or scope would otherwise be unclear. 
6. It should assist the indexer and searcher in the selection of the most appropriate terms needed to represent a particular subject  concept through its hierarchical and cross-reference structure.