Participants are the calling party (caller) and the called party (callee). Recall that in the VoIP world one may establish a session with a group of callees. In such a case we may have more than two participants. (Other common cases for SIP sessions with multi participants could be the addition of participants to an existing session either by calling them (dial out) or by have them dial in or by merging separate sessions to one (See (mainly) RFC 3911 and RFC 4579 (Conf-ID;isfocus;Join)))