Next: A Data Inventory Service for the Virtual Observatory . ... UCDs (Unified Content Descriptors) are metadata that have been used in the VizieR catalogue service since they were first developed in the ESO/CDS data mining project. ... It uses a new syntax, and a list of atomic keywords from which UCDs are built. ... 2003). ... Of course, the resulting list of words, as any standardization process, is somehow subjective (for example, only one word is kept when there are synonyms of some concept). ...