This identifies which internal diagnostic subsystem reported the card's last fatal (service-affecting) error, distinct from cardDiagNonFatalSource which tracks non-service-affecting faults. Because a fatal error typically forces the card out of service or into a reboot, knowing the source matters for root-causing an outage: a fault(3) source usually means a hardware exception crashed the card, while a redundancy(5) source could mean the fatal condition originated in the failover logic itself, which is a red flag for the standby card's reliability too. For example, cardDiagFatalSource reading fault(3) right before an unplanned card reboot tells the admin the crash was hardware-exception driven, not a scheduled or software-triggered event.