- a set of related keywords
- a type of the keywords
- a citation to the source (thesaurus) for the keywords
The ISO keywords are implemented to allow communities to use keyword dictionaries that are appropriate for their community and to connect to those vocabularies using a citation in the keywordThesaurus. Communities can also define appropriate keyword types by extending the MD_KeywordTypeCode codelist.
The ISO Metadata Standards include the capability to provide a citation to the source of the keywords. These citations are used when the keywords are coming from a shared vocabulary or keyword list. The NASA Global Change Master Directory (GCMD) provides a number of commonly used keyword lists. Citations to shared keyword vocabularies are good candidates for components.