Sometimes you will notice, if you are using multiple jabber clients at the same time, that one client will recieve messages and the other will not. This is Presence Priority.
Let’s say that you have a work computer, a home computer and a phone. You might wish that messages mostly go to your work computer, but otherwise to your phone or home computer. Then you can prioritize in your IM application e.g. 43 for the work computer, 42 for your phone and 41 for your home computer. This will be that 41 is the last available client that messages will be sent to (if the work and phone is not available).
This way you do not receive the same message multiple places, but only in your most relevant situation. If you put the priority to the same in both clients, both client will receive the message (if there is no other clients with higher priority that are available).