This counts frames this interface had to hold back briefly because it sensed the shared medium was busy just before it tried to transmit. On a full-duplex Carrier Ethernet port there is no shared medium to defer to, so any growth in this counter for the current, still-accumulating short interval bin on this interface again points toward an unexpected half-duplex condition on this specific link, the same family of issue flagged by TxCollPkts and TxLateCollPkts on the same port. For example, TxDeferPkts moving off zero for this in-progress short-interval bin on a port that has run clean for months is worth pairing with a check of that port's current duplex/speed autonegotiation result.