If the resource is a spatial data service, at least one keyword from Part D.4 shall be provided. If a resource is a spatial data set or spatial data set series, at least one keyword shall be provided from the general environmental multilingual thesaurus
For each keyword, the following metadata elements shall be provided:3.1. Keyword valueThe keyword value is a commonly used word, formalised word or phrase used to describe the subject. While the topic category is too coarse for detailed queries, keywords help narrowing a full text search and they allow for structured keyword search.The value domain of this metadata element is free text.3.2. Originating controlled vocabularyIf the keyword value originates from a controlled vocabulary (thesaurus, ontology), for example GEMET, the citation of the originating controlled vocabulary shall be provided.This citation shall include at least the title and a reference date (date of publication, date of last revision or of creation) of the originating controlled vocabulary.Part D.4 of the Regulation on metadata includes the Classification of spatial data services. The keywords are based on the geographic services taxonomy of EN ISO 19119. This taxonomy is organised in categories, the subcategories defining the value domain of the classification of spatial data services.