Annotation of content often contains references to semantic entities such as objects, events, states, places, and times. In order to ensure consistent descriptions (e.g. make sure that persons are always referenced with the same name) some kind of controlled vocabulary should be used in these cases. MPEG-7 provides a generic mechanism for referencing terms defined in controlled vocabularies. The only requirement is that the controlled vocabulary is identified by a URI, so that a specific term in a specific controlled vocabulary can be referenced unambiguously. In the simplest case, the controlled vocabulary is just a list of possible values of a property in the content description, without any structure. The list of values can be defined in a file accessed by the application or can be taken from some external source, for example the list of countries defined in ISO 3166. The mechanism can also be used to reference terms from other external vocabularies, such as thesauri or ontologies. Classification schemes (CSs) are a MPEG-7 description tool that allows to describe a set of terms using MPEG-7 description schemes and descriptors. It allows to define hierarchies of terms and simple relations between them, and allows the term names and definitions to be multilingual. Part 5 of the standard already defines a number of classification schemes, and new ones can be added. The CSs defined in the standard are for those description tools, which require or encourage the use of controlled vocabularies, such as
- technical media information: encoding, physical media types, file formats, defects
- content classification: genre, format, rating
- other: affection, role of creator, dissemination format