message type
Click on the red underlined text to get to the source
...
An IM application SHOULD support all of the foregoing message types;
if an application receives a message with no 'type' attribute or the
application does not understand the value of the 'type' attribute
...
... specialized applications (e.g., a multi-user chat service) MAY at
their discretion enforce the use of a particular message type (e.g.,
type='groupchat').
...
... Specifying a Message Type ...