Hi Igor -
This is not a direct response to your comments, but one thing I did recently with SSAP (which I think has largely gone unnoticed) is enter both the SSAP and Spectrum data models into a spreadsheet. This makes it much easier to get a uniform view of things like UTYPEs, UCDs, units, what is provided and what is missing, etc., than trying to work all this out from the documents. You can find this at
https://webtest.aoc.nrao.edu/ivoa-dal/ssap-keywords.xls
The spreadsheet should display in either Excel or OpenOffice.
In doing this I found some cases where things like the UTYPEs were inconsistent in the Spectrum document; these cases should be fixed in the spreadsheet, but may not yet have been resolved in the specification. Also, there are detailed examples of suggested UTYPE usage (both in queries and in spectrum datasets) in the output that comes back from the SSAP reference services. Some of the details of how this is done probably need further discussion.
I still need to go back and note any differences from our current specifications, but in considering further changes to Spectrum I suggest we try to reference both the specification and the spreadsheet.