An application must never just play a packet without inspecting its payload type, even if a single payload type has been negotiated via H.245 or similar protocols. New mechanisms, including
- transmission of DTMF digits (RFC 2833),
- comfort noise indication,
- forward error correction using redundant data,
- switching of encodings to take into account network conditions
may conveniently use the PT to indicate special packets, which an end application can ignore, if desired, ensuring backward compatibility. But this assumption is violated if an application blindly plays back all packets regardless of PT.
Also, in multicast environments, it is unlikely that every sender will use the same payload type.