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 this completed, frozen 24-hour historical 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 completed 24-hour bin on this port, scaling with TxPkts, would confirm that gap is framing overhead rather than a counting bug.