The definition of XMPP stanzas in XMPP Core  and XMPP IM  allows a <message/> stanza to include any number of child elements that define extended content. The fact that a message stanza may contain multiple instances of extended content can make it difficult for sending entities to know what is appropriate for inclusion in a message stanza and for receiving entities to know exactly how to process a message stanza.
Consider the following hypothetical example:
What to make of a message like this? The import seems to be that Romeo, being in a flirtatious mood (User Mood (XEP-0107) ) and currently located near Juliet's abode (User Geolocation (XEP-0080) ), would urgently (Stanza Headers and Internet Metadata (XEP-0131) ) like to meet with Juliet (message body) and proposes two convenient places (Feature Negotiation (XEP-0020) ) for an evening tryst, but no later than midnight (Advanced Message Processing (XEP-0079) ), and for good measure would like to transport the 85th packet of a file transfer. But how is Juliet's client supposed to figure that out? That is, what should her client present to the user? And should Romeo's client even send a monstrosity such as this?
To clarify such matters, this document introduces the concept of "message stanza profiles". Each profile defines a set of elements that are allowed for use, and each message stanza must only be processed under the rules of a single profile. Therefore, at best, Juliet's client in the above example would process the message for just one of its many intended purposes. At worst, the message would be rejected by Juliet's client as invalid. Romeo's client should not be sending such a monstrosity.
Elements used in a message stanza are either part of a standard profile (for example, <body> or <data xmlns='http://jabber.org/protocol/ibb'>) or the metadata profile (for example, <error> or <amp xmlns='http://jabber.org/protocol/amp'>). A message stanza SHOULD only contain profile elements that are part of the same profile. A message stanza MAY contain as many elements as necessary from the metadata profile.
A sender MAY mix elements of different profiles only for the purposes of fallback. For example, it may be useful to include a <body> element in a message that is not intended to be displayed as a text message, in case the recipient does not support the primary profile of the message.
A receiver MUST only process the message for a single profile. The above example message would be processed either as an IBB data packet or as an instant message, but never both.
We stipulate the following rules:
If a client receives a message stanza that combines multiple profiles, or the profile cannot be determined, the client MAY return a stanza error, which SHOULD be <not-acceptable/>.
A client might determine that a message cannot be assigned to any profile (e.g., a message stanza that is empty or that contains only whitespace , or that contains only unknown elements).
Profiles can be defined in whichever specifications define elements or processing behavior related to message stanzas. All such profiles SHOULD be registered in the Message Profiles Registry as described in the XMPP Registrar Considerations. However, a few profiles are defined here, and this specification might be kept up to date as further registrations are created.
The instant messaging (IM) profile is the "default" profile for message stanzas. For example, if a message stanza includes only elements that are defined for the 'jabber:client' namespace then it is in the IM profile. If a message stanza includes both IM profile elements and other elements, the IM elements should be considered a fallback and the profile should be determined based on the other elements if supported (e.g., a data form). A sending entity should limit the elements it includes to IM profile elements, unless the IM elements are a fallback.
The extended content defined in the following specifications is considered to be in the IM profile:
The metadata profile is handled differently than the other, "standard" profiles, but is defined as a profile for the purpose of consistency. Metadata elements are included to define how the message stanza shall be routed, delivered, or processed in transit or by the end recipient. Metadata elements shall not be used to determine which standard profile applies. If a message stanza includes only metadata elements, it can be considered to have no standard profile.
The extended content elements defined in the following specifications are considered to be metadata elements:
Since each message is unambiguously determined to be of a specific profile, implementations that use filtering to pass message stanzas to an appropriate handler (a very common XMPP implementation approach) need not be concerned with the filtering order. This is because only one handler should ever match on the filter expression.
This document adds no security concerns or consideration above and beyond those specified in the specifications to which it refers.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
A future version of this specification may call for the XMPP Registrar  to establish a registry of message stanza profiles, so that each relevant specification shall define which profile applies to extended content qualified by the relevant namespace.
The XMPP Registrar shall maintain a registry of message profiles.
In order to submit new values to this registry, the registrant shall define an XML fragment of the following form and either include it in the relevant XMPP Extension Protocol or send it to the email address <email@example.com>:
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This XMPP Extension Protocol is copyright © 1999 – 2018 by the XMPP Standards Foundation (XSF).
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This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).
The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <firstname.lastname@example.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
Errata can be sent to <email@example.com>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
8. A message stanza SHOULD NOT be empty or contain only whitespace, but some clients are known to generate such stanzas.
22. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
23. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
For consistency, defined Metadata Profile; specified that the registrar shall create a registry for message stanza profiles.
Initial published version; specified more granular profiles; renamed transmission elements to metadata elements.
Clarified that message profiles apply to sending entities as well as receiving entities.