This read-write enum determines which frame types of this protocol class get blocked on this specific port: all(1) blocks everything in the class, sr(2) blocks only source-routed frames, nsr(3) blocks only non-source-routed frames, and none(4), the default, allows everything through. An admin trying to stop a legacy or unwanted protocol from traversing a specific port without affecting other ports would set this to all(1) for that protocol's class on just that port. Selecting sr(2) or nsr(3) specifically lets an admin block one routing style of a protocol while still permitting the other, which matters in mixed source-route/transparent-bridged Token Ring environments.