Classification of Controlled Vocabularies

In understanding English, if we want to figure out what a word means, we might consult a dictionary or a glossary. Or we may use an etymology dictionary to track the history of a word. If we want to know how a term relates to other terms we might consult a thesaurus.

Like the vocabulary sources for the English language, controlled vocabularies for describing metadata can be classified by their purpose, their form, or their functionalities.

Classification by Purpose

Vocabularies may be defined by their ability to accomplish specific goals:

  • Discovery vocabulary: helps users find data
  • Usage vocabulary: assists in the interpretation of data
  • Semantic vocabulary: provides human-understandable meaning
  • Syntactic vocabulary: translates information into machine-readable format

Controlled vocabularies provide these abilities by

  • establishing the permissible terms to be used;
  • maintaining the proper and agreed-upon spelling of the terms;
  • clarifying terms for those who are new to the community; and
  • eliminating the use of arbitrary terms that can cause inconsistencies and confusion.

Classification by Form

To enable formal management, a controlled vocabulary can be organized structurally such that that it fits into one of these broad categories:

  • Flat: provides a set of required terms that may be used. Some flat controlled vocabularies will provide additional information about each term.
  • Multilevel: builds upon a flat controlled vocabulary by assigning each term to a category.
  • Relational: provides a set of terms and captures how they are associated with each other.

Classification by Functionality

Within the three broad categories that classify controlled vocabularies by form, there are sub-groupings that we will call “types.” The table below summarizes the relationships between the broad, form-based categories and their respective function-based types. The table defines the types and categories according to their minimum required characteristics.

Broad, Form-based Category

Functionality-based Type


Flat Controlled Vocabulary

Authority File

List of terms


List of terms and definitions within a specific domain


List of terms, definitions, and additional information


List of place names

Code List

List of codes (e.g., abbreviations) and definitions

Multilevel Controlled Vocabulary


Terms classified into categories

Subject Heading

Terms classified into categories, which may be broad classes

Relational Controlled Vocabulary


Set of terms and relationships among individual values

Semantic Network

Set of terms/concepts and directed relationships


Set of terms and relationships among terms, enhanced by additional information provided by rules and axioms.

Hybrid Classifications and the Real World

Not all controlled vocabularies fit neatly into one type; some may appear as hybrids or crossovers. Vocabularies rarely exist in a vacuum and evolve over time, causing the distinctions between the classifications to be muddied, either intentionally or unintentionally. In addition, vocabularies can fit multiple classifications.

Consequently, one controlled vocabulary might fit the definition of more than one type. For example, an ontology might also have many of the characteristics of a dictionary. Because of this ambiguity, the different types may be referred to generically as "vocabularies" or "controlled vocabularies," especially if they have hybrid characteristics.

Comparing and Understanding Classifications of Controlled Vocabularies

The guides in this section contain several articles to help you understand the distinction between classifications of controlled vocabularies and to examine some types side by side. Also, see the article Knowledge Organization Systems for more information.

Suggested Citation

Neiswender, C., Isenor, A., Montgomery, E., Bermudez, L., Miller, S.P. 2011. "Classification of Controlled Vocabularies." In The MMI Guides: Navigating the World of Marine Metadata. Accessed October 26, 2020.