This is the free-text description of the card's last fatal error, the fatal counterpart to cardDiagNonFatalStr. Because it's captured at the point of the crash, this text is often the most useful single field for a first-pass triage of an unexpected card outage, letting an admin describe the failure in the trouble ticket without waiting for a deeper code lookup. For example, cardDiagFatalStr reading Unrecoverable bus timeout on IOP interface immediately tells the admin to suspect the backplane connection rather than the card's own logic.