Hi Roy,
Great to see something practical happening. One comment - we do need to see the use of UCDs alongside the use of the Registry and data models (e.g. Pedro Osuna's very helpful reply which has indeed lifted my confusion about non-table data - or that bit of it anyway). Hence, when people provide implimentation of use cases, it would be nice to see:
Where/if UCDs are used in the Registry to select data-sets (and if only UCDs, not other entries e.g. spectral coverage, keywords, serve a particular purpose);
How UCDs are used (ideally in the context of a data model) to enable the processing steps in the execution of a query.
Use case - if you are sick of Brown Dwarfs read no further: http://wiki.astrogrid.org/bin/view/Astrogrid/BrownDwarfRegistryRequirements
The RegistryQuery steps use some things which could be UCDs like Proper Motion but they might also be key words. Specific colours are also used, but it might be sufficient to know that a catalogue contains optical and colour data.
The DataSetQuery/Evaluation steps use these UCDs above and others. The new UCD structure enabling the use of partial matches would be a great help as the workflow contains choices where you use a quantity (e.g. colour) if already defined but if not you derive it ("you" being a data processing software agent).
More examples could be drawn from the AstroGrid Ten...
cheers
a