The operations timing source for the ERX system. The ERX periodically
monitors the status of three timing sources, primary, secondary and
tertiary. If the systems current timing source fails, the system will
automatically downgrade to the next timing source. If the system is
configured to automatically upgrade (usdERXSysTimingDisableAutoUpgrade
is set to false(2)) the system will switch back to the higher timing
source when it becomes available.