(second of two messages)
These all have distinct meanings.
It means just that, "unknown". So for 199u, the 'u' may be replaced by any single digit.
[Note: we have not settled on 'u' as the "unknown" character. We just haven't found a better one yet.]
it means "known to be one of a set". So 2004-[01,02,03] means "January, February, or march of 2004, we don't know which but we know it's one of those".
A strict value, but this value may be wrong. 2004-06? means "it may be June 2004, but then, it might not."
It means just that, "approximate".
There has been discussion about whether and how (1) precision might be assigned to "approximate", and (2) probability might be assigned to "questionable. I hope we agree that qualification is not necessary for "unknown" and "uncertain".
If indeed there is a need to assign precision and probability to approximate and questionable then I propose the following basic approach.
First let's agree, can we, that most, in fact the vast majority, of approximate and questionable, will not need to assign precision or probability, they will be happy to simply assert that the value is approximate or questionable. Therefore we want an approach that allows this assertion in the simplest possible way without the burden of any complexity imposed by the qualification syntax. In other words if you simply want to assert june of 2004, approximately" it's '2004-06~' as currently in the draft spec.
I propose an extension mechanism, whereby whenever a '?' or '~' is encountered, it may be followed by an extension. The extension would be delimited in some fashion, and I am not proposing how at this point, but let's say for now we use ampersand. Then
2004-06~ means "june of 2004, approximately"
2004-06~&123abc& means "june of 2004, approximately, with a precision of '123abc'"
So whenever ~ is encountered, if followed by & then a precision extension follows, terminated by the next & (and I have no idea what '123abc' means, someone will have to come up with a framework for these extensions)
Similarly for questionable, whenever a ? is encountered, if followed by & then a probability extension follows.