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, и P. Charland. Models in Software Engineering: Workshops and Symposia at MoDELS 2006, Genoa, Italy, October 1-6, 2006, Reports and Revised Selected Papers, том 4364 из LNCS, стр. 56--65. Springer Berlin/Heidelberg, (2007)
G. Guizzardi, и G. Wagner. CAiSE Workshops, стр. 129-143. Faculty of Computer Science and Information Technology, Riga Technical University, Riga, Latvia, (2004)