This OID stores the destination port number (TCP or UDP) for data flowing through a quality-of-service reservation session. It tells you which port on the receiving device the traffic is destined for; a value of zero means the protocol being used doesn't use ports (like IPsec). An operator would check this when troubleshooting why traffic on a particular port isn't getting the expected reserved bandwidth, or when verifying that a QoS session was set up correctly for a specific application.