This counts BTUs dropped because the card ran out of internal buffer memory to hold them. This is a congestion indicator specific to this card's buffering capacity; sustained growth here during otherwise normal traffic levels suggests the card's buffer pool is undersized for its current traffic profile, which is a capacity-planning signal distinct from a pure bandwidth-exhaustion problem. For example, cardInNoBufferBtus rising during a burst of many small PVCs' traffic converging on this card at once suggests buffer exhaustion from microbursts rather than a sustained bandwidth shortfall.