The focus of this specification is instant messaging applications and so those (and only those) &MESSAGE; stanzas used for instant messaging SHOULD be delivered as Carbons.
The following is the set of rules that a server implementation SHOULD use to determine which messages should be carbon-copied. Future specifications MAY add or override rules, though they are generally advised to use the <private/> element.
+A &MESSAGE; is eligible for carbons delivery if it does not contain a <private/> child element and if at least one the following is true:
As the above is an implementation detail of servers, clients MUST NOT rely on the server implementing a particular set of rules for which messages are eligible for Carbons delivery.
Future specifications may have more precise requirements on which messages need to be eligible for carbons delivery; such future specifications will provide their own discovery and negotiation mechanisms, such that a client negotiating Carbons using the protocol defined in this specification will cause the server to consider messages eligible for Carbons delivery based on the requirements described herein.
Note: previous versions of this specification limited eligible messages to those of type "chat" - however, this was generally found to be inadequate due to the proliferation of type "normal" messages used in instant messaging.
+A server implementation can choose to advertise full support of all the rules in ยง6.1 by including the "urn:xmpp:carbons:rules:0" feature in its service discovery information. If that feature is advertised, the rules above must be treated as REQUIRED and not merely as RECOMMENDED.
+Accordingly, if this feature is advertised, a client MAY rely on the server supporting this exact set of rules.
+