On this issue, I’d like to note that the last part of the sentence for Recommended class reads in full:

“…sender of data MUST provide information about instances of the class, if it is available.”

So the intention is that, if the information is available, the information MUST be provided. This is a stronger statement than SHOULD – with SHOULD the sender can decide not to provide the information, even if it is there.

In my understanding, "recommended" classes / properties are also meant to encourage (meta)data providers to include this information, also if currently they don't have it.

So, my concern is that if we say "must if available", this may be implicitly interpreted: "if I don't have it, there's no requirement why I should provide it".

For the reason above, I think the "SHOULD" must be used, possibly combined with "must if available" - e.g.,

a sender of data SHOULD provide information about instances of the class; a sender of data MUST provide information about instances of the class, if such information is available.

BTW, "must if available" is what in INSPIRE falls under the notion of "conditional obligation", i.e., "mandatory under given conditions" (e.g., if the information is available).

Analysis: Discussion has taken place, and new text has been proposed.

Proposal: Modify the text for Recommended class to “a sender of data SHOULD provide information about instances of the class; a sender of data MUST provide information about instances of the class, if such information is available.”