This counts how many times the CBR interface's receive FIFO filled beyond capacity, meaning cells arrived faster than the card could hand them off to the TDM side and some were dropped. An admin watching this climb on a specific lport knows that circuit's egress clocking or buffer sizing can't keep pace with its assigned traffic, and left unaddressed it will eventually trigger a lportCBRLineDataError trap. For example, a burst of 500 overflow events right after a bandwidth reallocation would point toward a card that is now oversubscribed relative to its FIFO depth.