Dear @martinjuckes and JimBiardCics
Thank you for the large amount of time you've spent on this issue. Rereading it 
I find I'm not certain what is the current question being debated. This issue 
started as a defect, but it could only be accepted as such if it wasn't 
challenged i.e. it concluded as a short discussion about correcting something 
where the words didn't match the intended meaning. It's evident from the length 
of the discussion that the intended meaning wasn't clear, and that requires us 
to decide what it should be, as as an enhancement to the convention. Should we 
proceed to do so in this issue, and relabel it?
My opinion is still that a string variable `X(X)` should not be allowed, 
because we do not permit string-valued coordinate variables (meaning 
string-valued dimension coordinate constructs in terms of the data model) and 
that is what it looks like. Therefore it would be confusing to allow it even as 
an auxiliary coordinate variable. This is simply to avoid misunderstanding.
I think it's a different issue whether we should improve the language in the 
conventions document. That also wouldn't be a defect, but an enhancement. It 
might indeed be better to use words like "shall", "should" and "may" 
consistently, state the sense in which we use them, and not use alternatives 
anywhere. Richard Hattersley asked about this too, years ago. To make that 
enhancement would clearly require a very careful checking of the document to 
list all the changes required correctly.
As a separate enhancement, I would also propose diverging from the NUG 
convention, to take advantage of the CF data model that we have now agreed, by 
using the phrase "dimension coordinate variable" for `X(X)` (where `X` is 
numeric), instead of the NUG "coordinate variable", which has *always* been a 
source of confusion in the CF document. If we ever need to refer to something 
that could be either a dimension coordinate variable or an auxiliary coordinate 
variable, we can call it a generic coordinate variable (following Hassell et 
al., 2017, about the data model). Thus we would never use the unqualified 
phrase "coordinate variable".
Cheers
Jonathan

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