The value of tmnxNatPlcyToSubRetention specifies the subscriber retention timeout, the time a NAT subscriber and its associated IP address is kept after all hosts and associated port blocks have expired. If a NAT subscriber host appears before the retention timeout has elapsed, it will be given the same outside IP address. A non-zero value is only allowed while the value of 'tmnxNatPlcyPurpose' is 'nat'.