This read-write, non-volatile FrequencyKhz attribute is meant to define an alternate downstream frequency for the ComPort, defaulting to the invalid value 0, but the description explicitly flags it as a to-be-determined feature not available in the 2.3 release. An admin needs to know this is not yet a functioning failover mechanism, so writing a value here on 2.3-era firmware won't actually change downstream failover behavior. For example, a script that sets stuDnstrmAltFreq expecting automatic downstream failover would see no operational effect on a ComPort still running 2.3-release software.