Dear all

I'd like to repeat my earlier points that

  * We should make use of the existing list, namely the conformance document, 
for the purposes being discussed here - I don't think we need a new list.

  * We don't have to distinguish positive and negative categories, because they 
are logically related: prohibition = negative requirement, deprecation = 
negative recommendation.

Does anyone disagree with those points, I wonder?

Which of these categories should be used if we discover a *flaw* in the 
convention, which allows metadata to be produced that can't be interpreted 
correctly or reliably (as in 
https://urldefense.us/v3/__https://github.com/cf-convention/cf-conventions/issues/314)?__;!!G2kpM7uM-TzIFchu!hyhLEMLDzUlm9Sg9Um2Auq80CWA3cMBoc6vBg7NtxWJIfNXhgjtZu_8brtb6g5m6xi6Q6D-B8DM$
  The rules say "deprecate" but I think now that's too weak. We should 
*prohibit* the use of the flawed convention (not the whole version, just the 
affected part) for writing new data (but also reassure users that existing data 
isn't being invalidated).

I appreciate the arguments about the need for a further distinction, and I 
agree this could help in other cases. I suggest that we need to distinguish 
between recommendations which are made for good practice (and could remain for 
ever), and recommendations which are made because there are alternative ways to 
do something where one is preferred and the other might be abolished in future. 
That is, we would have three categories in the conformance document, rather 
than two. An example of a good-practice recommendation in the conformance 
document is "The name of a multidimensional coordinate variable should not 
match the name of any of its dimensions." We do not envisage making this a 
requirement.

In general, we do not try to foresee the future of the CF convention, so I 
think most of the current recommendations are for good-practice. There should 
only be a few where we think it's really likely that we are going to abolish 
something in future. I note that, up to now, we have not abolished anything. 
One reason for that is because past data continues to exist for a long time. 
Therefore it's hard to withdraw support for any feature in data-reading 
programs without causing inconvenience, although you can in data-writing 
programs. I know that you can always inspect the Conventions attribute, but 
most user programs don't pay attention to that, I imagine, and we should avoid 
making things awkward for users.

Hence I would suggest identifying which current recommendations in the 
conformance document, or which should be in it but aren't, ought to be promoted 
to a new category of things which really might become requirements/prohibitions 
in future. What could this new category be called? Warnings?

Best wishes

Jonathan


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