In the context I mentioned it would be how it is commonly done in Linked Data where e.g.
http://purl.org/goodrelations/v1#* namespace →
http://www.heppnetz.de/ontologies/goodrelations/v1#* html specification docs
And for FEP’s, see FEP-888d: Using w3id.org/fep as a namespace for extension terms and for FEP documents (where I’ll have an issue with the non-descriptiveness of the namespace, that I will comment about on that thread).
Update: I added feedback on fep-888d regarding regarding descriptiveness and intuitive naming of namespaces and namespace prefixes. Two observations in the reply are relevant to copy here:
A FEP is a formal specification of a functional building block for the creation of interoperable apps.
Interoperability on the Fediverse is not about apps, but on finding common denominators where compliance exists.