This counter tracks how many transmit-side VBUS backplane buffer overruns have occurred for this module, meaning frames the module tried to send across the backplane but had to drop because the transmit buffer was full. A steadily climbing value here indicates the module is trying to push more traffic onto the backplane than the VBUS fabric can absorb, which an admin would investigate as a sign of oversubscription or a backplane performance bottleneck on that card. An NI card showing chasModuleVbusTxDiscards climbing by hundreds per minute under heavy uplink traffic points to backplane congestion, not just a downstream port issue.