The definition of XMPP stanzas in &xmppcore; and &xmppim; 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:
Elements used in a message stanza are either profile elements (for example, <body> or <data xmlns='http://jabber.org/protocol/ibb'>) or metadata elements (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 metadata elements as desired.
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.
If a message stanza is received that combines multiple profiles, or the profile cannot be determined, the receiving client MAY return a stanza error, which SHOULD be ¬acceptable;.