You use this policy when you want to lock transmissions to only one link. Regardless of how many other links are available, Qnet will latch onto the one interface you specify. And if that exclusive link fails, Qnet will not use any other link.
Why would you want to use the exclusive policy? Suppose you have two networks, one much faster than the other, and you have an application that moves large amounts of data. You might want to restrict transmissions to only the fast network, in order to avoid swamping the slow network if the fast one fails.