This identifies which internal diagnostic subsystem, such as the power-on test, background test, fault handler, frame-heap manager, or redundancy logic, logged the most recent non-fatal error on this card. Because non-fatal errors don't take the card out of service, this field is what lets an admin decide how worried to be: a source of background-diagnostics(2) reporting an intermittent issue is far less urgent than one flagged by redundancy(5), which suggests the failover mechanism itself might not work when needed. For example, seeing cardDiagNonFatalSource read i960-data-structures(8) points a technician toward a corrupted internal data table on the card's i960 processor rather than a hardware I/O fault.