Gruber said he saw four different ways of adding meta-tags to material, ranging from the loosest to the most strict:
1.Folksonomy (informal, user-defined)
2.Controlled vocabularies (the user must deploy a set of defined terms)
3.Taxonomy (Pre-defined terms in which specific terms are subsets of more general terms)
4.Ontology (A rich set of relationships is mapped out among all the terms)
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J. Baumeister, T. Kleemann, и D. Seipel. FLAIRS'07: Proceedings of the 20th International Florida Artificial Intelligence Research Society Conference, стр. 524-529. AAAI Press, (2007)
K. Lüttich, B. Krieg-Brückner, и T. Mossakowski. FORMS/FORMAT 2004 -- Formal Methods for Automation and Safety in Railway and Automotive Systems, стр. 109-119. (2004)
V. Alexiev. Digital Presentation and Preservation of Cultural and Scientific Heritage (DiPP2012) conference (Invited report), Veliko Tarnovo, Bulgaria, (сентября 2012)