This is a running total of all octets transmitted out of this physical port, including framing overhead. Comparing this to pportInOctets on the same port over time reveals whether traffic on this link is roughly symmetric or heavily skewed in one direction, which matters for capacity planning on that specific circuit. A DS1 port whose pportOutOctets consistently approaches the line's theoretical 1.544 Mbps ceiling is a candidate for an upgrade to a higher-rate port.