SUBSCRIBE, NOTIFY and PUBLISH methods in SIP are responsible for carrying presence information to a target destination. Message headers (Request-line, To, From, Contact etc.) indicate SIP entities that are identified by a URI. This document clarifies the indication of the message headers and provides a guideline on URI usage in the header fields. The authors hope that this document will be useful for SIP/SIMPLE implementers, interoperability testers, designers, and protocol researchers.