This read-only counter tracks how many all-ones filler cells the channel inserted into its output stream to cover for an egress buffer underflow, a stopgap measure to keep the TDM line's timing continuous even when real payload data ran out, and it is not supported on CESM-8P. An administrator sees a rising count here directly alongside cesBufUnderflows, since each underflow event typically triggers insertion of one or more of these filler cells. Frequent insertion of all-ones filler cells signals unresolved jitter or buffering problems degrading the quality of the emulated circuit.