This counts bus transfer units received by this card over the internal cell bus, functioning as this card's inbound half of the same BTU accounting seen elsewhere in this MIB for other cards. Because BTUs represent the chassis-internal transport unit rather than external network traffic, tracking this on a per-card basis lets an admin isolate whether a backplane throughput problem is specific to this card's bus connection rather than affecting the whole chassis backplane. For example, a card showing an unusually low cardBTUsFromBus rate compared to sibling cards handling similar external traffic volumes suggests a backplane connection issue unique to that card.