This object identifies which session index (from the live sessions table) the failure record corresponds to, but per the desc it is only meaningful when crasSessFailType for that row is operationalFailure - for setup failures the value is undefined and should be ignored. An admin investigating an operational failure uses this index to pivot from the failure-history row back to whatever additional session details still exist for that former session, such as its traffic counters, before it dropped. Trying to look up this index on a row marked setupFailure would be a wasted step, since no such session ever fully came into existence.