This Counter64, in Protocol Data Units, counts application-layer PDUs whose protocol operation, such as the HELO command in SMTP, was disallowed by the firewall's local security policy, and requires the firewall to be doing deep packet inspection of application traffic to be meaningful. An admin uses it to detect clients or servers attempting application-protocol commands that policy has decided to forbid. A rising count on this counter after enabling a new application inspection policy would confirm that policy is actively catching disallowed protocol operations.