This is a running byte counter of everything received on this card's ports, counted at the line level including framing overhead, not just payload. Because framing bytes are included, this counter runs slightly higher than a pure payload-throughput calculation would, which matters when an admin is trying to reconcile SNMP-reported utilization against a billing system that counts payload bytes only. For example, comparing the delta of cardInOctets between two polls against the card's total line rate over that interval gives a rough total, not just payload, utilization percentage for capacity planning.