I really like this proposal, there should be a standard for this in my opinion.
In detail, what I like more about Reinzor’s message definitions, is that there is a shared CategoricalDistribution.msg, which takes the role of the ids+scores combination in Classification2D.msg
I think using the CategorialDistribution reduces a little bit on complexity on the user end, but the difference is very small. Another benefit is that the message definition is reusable.