For FORM_TYPEs that are not registered with the XMPP Registrar, the field name SHALL follow the extension fules described below and the field typically will not be registered with the XMPP Registrar.
The "namespace" of a field is assumed to be inherited from the FORM_TYPE. When an organization or project defines a field that is used in the context of a FORM_TYPE it does not manage (e.g., a non-XSF field contained in a form whose FORM_TYPE is managed by the XSF, or a third-party field contained in a form whose FORM_TYPE is managed by some other organization), the name of the field MUST be namespaced with a URI controlled by the extending organization or project, where the namespacing mechanism MUST be &clark;, e.g., a field name of "{http://example.com/pubsub}time_restrictions".
+For reasons that are lost in the mists of time, some XMPP extension protocols produced by the XSF, such as &xep0045; and &xep0060;, prefix their field names with strings like "muc#" and "pubsub#". There is no good reason to apply that convention to new XSF extensions. Indeed, there is even no good reason to apply that convention to the names of new fields defined by the XSF for those existing XSF extensions; however, the practice is harmless for those existing extensions (since a string such as "{http://jabber.org/protocol/pubsub#subscribe_authorization}pubsub#subscriber_jid" can be considered equivalent to a string such as "pubsub#subscriber_jid"), and this document does not actively recommend deprecating the convention.
Note: Older versions of this specification mandated that unregistered field names had to begin with the prefix "x-". In accordance with &xdash;, that mandate has been removed. However, existing "x-" field names are acceptable and can be registered with the XMPP Registrar as described above.