This counter tracks flow control frames received by the device, explicitly including both XOFF and XON frames. This is the mirror of the Tx flow control pause counter -- here, some downstream device is telling the CDD-564 to pause its own transmissions, worth correlating with the router's outbound Ethernet counters to see if it's being unnecessarily throttled by an overly aggressive flow-control policy on the attached switch. If this counter climbs steadily while the good-frames-transmitted growth slows, that pairing suggests the attached switch is pausing the router more often than the actual traffic load would justify.