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)
J. Rilling, Y. Zhang, W. Meng, R. Witte, V. Haarslev, and P. Charland. Models in Software Engineering: Workshops and Symposia at MoDELS 2006, Genoa, Italy, October 1-6, 2006, Reports and Revised Selected Papers, volume 4364 of LNCS, page 56--65. Springer Berlin/Heidelberg, (2007)
G. Guizzardi, and G. Wagner. CAiSE Workshops, page 129-143. Faculty of Computer Science and Information Technology, Riga Technical University, Riga, Latvia, (2004)
A. Gangemi, C. Catenacci, M. Ciaramita, and J. Lehmann. Proceedings of SWAP 2005, the 2nd Italian Semantic Web Workshop, Trento, Italy, December 14-16, 2005, CEUR Workshop Proceedings, (2005)
W. Lepuschitz, A. Lobato-Jimenez, E. Axinia, and M. Merdan. Industrial Applications of Holonic and Multi-Agent Systems: 7th International Conference, HoloMAS 2015, Valencia, Spain, volume 9266 of Lecture Notes in Artificial Intelligence, Springer, Cham, (2015)