This is a running byte counter, at the line level including framing overhead, of everything transmitted out of this card's ports, the transmit-side counterpart to cardInOctets. Comparing this counter's rate against cardInOctets on the same card reveals whether the card is roughly balanced, typical for a transit trunk, or heavily asymmetric, typical of an access card serving mostly one-directional traffic like video distribution, informing capacity decisions differently for each case. For example, cardOutOctets rising far faster than cardInOctets on an access card suggests it's primarily serving outbound content delivery rather than symmetric traffic.