This counts Change of Authorization failure notifications the gateway has sent, each one reporting that an incoming authorization-change request could not be carried out. A rising count means the gateway is rejecting change attempts, often because the CoA references a session or attribute the gateway no longer recognizes, leaving subscribers stuck on their old plan or QoS despite billing already recording the change as done. A burst of CoA failures right after subscribers roam to a different gateway would fit this pattern, since the session IDs referenced by the CoA no longer exist on the new gateway.