This read-write value sets the portion of the port's data rate reserved for ATM QoS Class 3 traffic, a mid or lower-priority tier used for business-critical data traffic that needs some guarantee but not the strict guarantees of the top two classes. An admin sizes this based on how much guaranteed throughput a class-3 application, such as transactional business data, actually needs, distinct from the dynamic or best-effort behavior of class 4. For example, a bank's transaction-processing traffic assigned to QoS class 3 would have its guaranteed minimum throughput defined by lportAtmDataRateQoS3, ensuring it is not crowded out entirely by best-effort class-4 bulk transfers.