This read-only NncExtCounter64 reports the number of CLP 0 (cell-loss-priority 0, i.e. high-priority) cells discarded due to congestion at this point, since the interface started. An admin watches this specifically because CLP 0 cells are meant to be protected from discard under normal congestion control, so any loss here is more serious than ordinary CLP 1 discards. A nonzero nncAtmCongStatsClp0CellsDiscarded would tell an admin even high-priority traffic is being dropped, indicating severe sustained congestion at this point.