This counts received frames from the satellite WAN interface whose HDLC address doesn't match any address the modem is configured to accept, and also doesn't match the broadcast address; it stops incrementing for such frames if 'frame bypass' is enabled. A rising value here usually indicates the link partner is transmitting with a different or misconfigured HDLC addressing scheme, a link-configuration mismatch rather than ordinary line noise. For example, a sudden jump right after switching to a new satellite hub could mean the new hub is transmitting with a different HDLC address than this modem expects.