This counts frames the card received over the internal cell bus that had to be discarded because one or more of the BTUs making up that frame never arrived. Because a frame is reassembled from multiple BTUs, a rising count here specifically indicates the backplane is dropping or losing individual BTUs mid-transit rather than an external line error, which is a distinct fault domain an admin should trace to the chassis backplane or bus interface rather than the card's WAN-facing ports. For example, cardIncompleteFramesFromBus rising while all external line-error counters on the same card stay flat points the investigation toward the chassis backplane rather than the card's network-facing interfaces.