IMS Communication Service Identifier (ICSI) is used to route the service signaling to the correct Application Server, identify a proper terminal (UA) and address a proper application inside the terminal.
A SIP Communication Service is an aggregation of one or several media components and a particular service logic. The Communication Service Identifier (CSID) identifies the communication services making it possible for all entities in the networks (originating and terminating side) to evaluate the different possible protocol elements in order to determine which communications service is requested, and to arrive at the same result.