This records the highest severity level this specific fault has ever reached since it was first raised, even if its current severity has since dropped. An admin reviewing fault history uses this to see whether a fault that currently looks minor actually spiked to critical at some point, which matters for post-incident review even after the immediate symptom has calmed down. A fault currently showing severity minor but a highest-severity of critical tells the admin this condition was worse earlier and may be worth investigating even though it looks fine right now.