SIP MESSAGE has nothing to do with SIP SIMPLE suite of standards.
SIP SIMPLE is a suite of standards for session based IM and file transfers using MSRP media plane and Presence. Presence requires contacts management for which XCAP server is used. XCAP is built on top of an HTTP server by serving XML documents defined for contacts and policies used by Presence. These documents can be changed by a SIP client with XCAP support.
More information about the SIP SIMPLE standards are found here: