This counter tracks how many BTUs (the switch's internal cell/frame transfer units carried across the backplane Cell Bus) arriving at this card were dropped because the card had no free BTU buffer to hand back in exchange, per the description. A rising count here means this specific card's BTU pool is being exhausted faster than the backplane can replenish it, which on a Cascade switch shows up as intermittent cell/frame loss on that card's ports even though the physical link itself looks fine. For example, seeing this counter jump from 0 to several thousand over a busy hour on a heavily trunked ATM card would point to backplane buffer starvation on that specific card rather than a wiring or carrier problem.