Is the IdentifierRS well specified?

[jonblower]

• Why isn't the id compulsory for individual identifier objects?
• Why isn't identifiers compulsory?
• Spec mentions description for targetConcept but not id (but examples have id and not description)

[letmaik]

• Why isn't the id compulsory for individual identifier objects?

Why should it? Sometimes there may not be an id (which is typically a URI).

• Why isn't identifiers compulsory?

The idea is that similar to a GeographicCRS it may be enough to specify "id": "https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2" which established the space of all identifiers. Inlining them is equivalent to inlining details of the CRS like axes and datum. It depends on the use case. Sometimes it makes sense to help the client out, sometimes the client may know all this already and just needs to know which reference system it is (the id).

• Spec mentions description for targetConcept but not id (but examples have id and not description)

Yes, I think that's an oversight. id should be added as optional field.