Currently, we allow an MF2 implementation (or its users) to extend the spec in two different places:

• custom functions
• private-use annotations

Of these, the functions may be defined in an XML registry, but we do not provide a place for the definition of private-use annotations.

Could we rename the registry to be called a "schema" instead, and include in it their definition as well? I don't think we'd necessarily need to say anything about how they are to be defined, but just provide a place that we leave empty ourselves.

This would allow for such a schema to be made available by an implementation, and for its URL to act as an identifier for users. Such an identifier would be useful in particular in the context of eemeli/message-resource-wg#14.