It means that due to the age of the vehicle, or the fact that the milage has gone over the mechanical limits of the odometer,or that the mileage was reported as unknown, that the state no longer tracks the mileage. Basically means the mileage on the odometer is not necessarily accurate, and cannot be proven. Has nothing to do with motor replacement. That can only be proven by a reciept for installation, or reciepts for components bought, if the guy did the motor himself.