IETF RFC 5364 PDF

In certain types of multimedia communications, a Session Initiation Protocol (SIP) request is distributed to a group of SIP User Agents (UAs). The sender sends a single SIP request to a server which further distributes the request to the group. This SIP request contains a list of Uniform Resource Identifiers (URIs), which identify the recipients of the SIP request. This URI list is expressed as a resource list XML document. This specification defines an XML extension to the XML resource list format that allows the sender of the request to qualify a recipient with a copy control level similar to the copy control level of existing email systems.