A few interesting excerpts from the document explaining the context and the rational:
"Astronomical information of relevance to the Virtual Observatory (VO) is not confined to quantities easily expressed in a catalogue or a table. Fairly simple things such as position on the sky, brightness in some units, times measured in some frame, redshifts, classifications or other similar quantities are easily manipulated and stored in VOTables and can currently be identified using IVOA Unified Content Descriptors (UCDs). However, astrophysical concepts and quantities use a wide variety of names, identifications, classifications and associations, most of which cannot be described or labelled via UCDs.
There are a number of basic forms of organised semantic knowledge of potential use to the VO. Informal “folksonomies” are at one extreme, and are a very lightly coordinated collection of labels chosen by users. A slightly more formal structure is a “vocabulary”, where the label is drawn from a predefined set of definitions which can include relationships to other labels; vocabularies are primarily associated with searching and browsing tasks. At the other extreme are “ontologies”, where the domain is formally captured in a set of logical classes, typically related in a subclass hierarchy. More formal definitions are presented later in this document.
An astronomical ontology is necessary if we are to have a computer (appear to) “understand” something of the domain. There has been some progress towards creating an ontology of astronomical object types to meet this need. However there are distinct use cases for letting human users find resources of interest through search and navigation of the information space...""As the astronomical information processed within the Virtual Observatory becomes more complex, there is an increasing need for a more formal means of identifying quantities, concepts, and processes not confined to things easily placed in a FITS image (Flexible Image Transport System), or expressed in a catalogue or a table. We propose that the IVOA adopt a standard format for vocabularies based on the W3C's Resource Description Framework (RDF) and Simple Knowledge Organization System (SKOS). By adopting a standard and simple format, the IVOA will permit different groups to create and maintain their own specialised vocabularies while letting the rest of the astronomical community access, use, and combine them. The use of current, open standards ensures that VO applications will be able to tap into resources of the growing semantic web. Several examples of useful astronomical vocabularies are provided, including work on a common IVOA thesaurus intended to provide a semantic common base for VO applications."