This is described as the total count of octets transmitted on this interface for this bin, alongside the plain TxBytes counter for the same port. Given Ciena's PM tables typically split a plain payload byte count from a total-including-overhead figure, the most likely reading is that TxTBytes adds preamble/inter-frame-gap overhead that TxBytes excludes for the current, still-accumulating short interval bin on this port -- worth confirming against the vendor guide before using the two interchangeably in a throughput report. For example, a small but consistent gap between TxBytes and TxTBytes for this in-progress short-interval bin on this port, scaling with TxPkts, would confirm that gap is framing overhead rather than a counting bug.