What I want to permit is evolution in technology for devices that doesn't limit our ability to store additional information that may be provided by devices. Suppose we get a new type of CGM that wants to store not only the calculated BG value, but the raw reading of whatever it's actually measuring? I wouldn't want that device to be dropping two readings every time, just so that it could store the extra value. It should be able to declare a subtype of a BG reading and add the extra field.

I see your point about the query performance, and I *think* I understand your subtag idea. Can you give me an example, please, perhaps along the lines of the one I just defined?