More about Service Broker priority in SQL Server 2008.
The books online states, when setting a initiator/target priority as local service name/remote service name that the priority affects:
1. Sends from the initiator queue
2. Receives from the initiator queue
3. Getting the next conversation group from the initiator queue.
And specifies the mirror image priority for target/initiator as local service name/remote service name affects:
1. Receives from the target queue
2. Sends from the target service
3. Getting the next conversation group from the target queue
So, in my previous example, where
The initiator/target as local service name/remote service name is priority 7
The target/initiator as local service name/remote service name is priority 6
Let's send a message from the initiator that "sticks" in sys.transmission_queue. Suppose that either the network is slow or we don't have the appropriate setup. But the message is floundering in the transmission queue. Already SENT, waiting to be delivered.
"select * from sys.transmission_queue;" shows the message BEING SENT (and transmitted) at priority 7. And, when the message is finally DELIVERED, the priority in the receiveq "select * from receiveq" is priority 6.
Normally, you'd likely make both priorities in the initiator/target pair the same to avoid confusion, but you can make them different for greater granularity. And it DOES help to illustrate where and when priority is used.