This counter records the number of priority-tagged frames discarded on this port due to the port's Acceptable Frame Policy, which restricts what kinds of tagged frames it will accept. A rising count tells the admin that devices are sending VLAN-priority-tagged frames the port's configured policy rejects, which could mean either the policy is correctly enforcing untagged-only access or that a device is misconfigured to tag frames it shouldn't. For example, if a newly connected device starts sending 802.1p priority-tagged traffic to an access port configured to accept only untagged frames, cmEthernetTrafficPortStatsAPFD would begin incrementing.