This is a Counter32 tallying bytes transmitted on a Generic HDLC bundle since the device last booted or the counter was last cleared, as its name indicates. An admin uses it to track cumulative outbound traffic on the bundle across a long time window, useful for capacity trending. For example, a steadily climbing nngenHdlcStatsBytesTxLastBootClear alongside a stalled nngenHdlcStatsBytesRxLastBootClear could indicate the far end has stopped acknowledging traffic.