There are basically two objectives for having a controlled vocabulary:
a) to promote the consistent representation of the subject matter of documents by indexers, thereby avoiding the dispersion of related documents, through control of synonymous and nearly synonymous expressions and by distinguishing among homographs; and
b) to facilitate the conduct of a comprehensive search, by bringing together in someway, the terms that are most closely related semantically.
The first of these objectives is achieved by controlling the terminology in various ways. First, the form of term is controlled, whether this involves grammatical form, spelling, singular and plural form, abbreviations or compound form of terms. Second, a choice is made between two or more synonyms, near-synonyms and quasi-synonyms. Third, homographs are distinguished. The control of synonyms is achieved simply by choosing one of the possible alternatives as the 'preferred term' and referring to this term (by using 'see' or 'use' references) from the variants under which certain users may be likely to approach. It should be obvious that the synonym selected as the preferred term (i.e. the searched for) must be the one under which the majority of users are likely to look first. Sometimes 'quasi-synonyms' are treated in the same way as synonyms (i.e. one is chosen and the reference is made from the other). The term 'quasi-synonyms' is not very precise. Many authors consider the quasi-synonyms as the antonyms that represent opposite extremes on continuum values. An example is the pair of word - 'roughness' and 'smoothness'. Clearly, 'roughness' may be regarded as merely the 'absence of smoothness', and vice versa.
The controlled vocabulary also distinguishes among homographs (i.e. words with identical spelling but different meanings), usually by means of a parenthetical qualifier or scope note. Thus CRANE (Bird) tells us that the term is to be used exclusively for a type of birds and not as a lifting equipment or any other possible context. By controlling synonyms, near synonyms and quasi-synonyms, and by distinguishing among homographs, the vocabulary control device avoids the dispersion of like subject matter and the collocation of unlike subject matter. In this way, it helps to achieve the objectives of consistent representation of subject matter in indexing and searching.
The second objective of vocabulary control is to link together terms that are semantically related in order to facilitate the conduct of comprehensive searches. For a hierarchically related (in a formal genus-species relationship) term, it will also reveal semantic relationship across hierarchies.