Minor "So a key design goal of XDI is to support the requirements of both persistent and reassignable forms of identification; to provide precise means to map between them; and to make it syntactically unambiguous which form is being used in which context."
Shouldn't the 1st sentence say "... to support the benefits of both..." ?
Or remove "the requirements of".
Comment: unlike most other design goals, it is not clear to me how we achieved the aforementioned goals. In particular, I'm not sure where reassignable identifiers are used in XDI? Is it through dictionaries?
Any portability of other definitions (of identifier conventions, dollar words, etc.) would be via XDI dictionary definitions. However, a single application could simply use its schema by convention in its code, without having dictionary definitions for portability, if it doesn't need to communicate this to other applications.
I think 1st sentence means "we previously specified a list of requirements, and this is what does this requirement".
= and + names are persistent. This is enforced by the registry, not by anything in XDI syntax itself. Any other identifiers are potentially reassignable, though applications will likely have conventions for considering additional classes of identifiers as permanent in that application's context.