If the username starts with admin or admin-, the subrequest handler responds with a status code of 200. This response allows any client request to proceed to the registry via the original handler (location /). A match on this condition effectively grants full read-write access to the registry.
If the username does not start with the admin prefix, but the HTTP request method is GET or HEAD, the subrequest handler responds with 200 status. This response allows the client request to proceed to the upstream registry. A match on this condition effectively grants read-only access to the registry.
In any other case, the subrequest handler responds with a 403 Forbidden. The original request handler aborts further processing and returns the 403 response to the client. A match on this condition prohibits the client request from reaching the registry. In effect, this condition prevents all anonymous user access and prevents anything but GET and HEAD access (i.e., read-only access) by non-admin users.