The id of the backup.
Reason for failure if the aggregated state is 'FAILED'.
What each physical tenant reports for this backup id, ordered by physical tenant id. Every targeted tenant is listed, including the ones reporting DOES_NOT_EXIST.
The state aggregated over every targeted physical tenant, whether the backup id was looked up directly or listed. A tenant holding nothing for this id counts as DOES_NOT_EXIST, so the aggregate is INCOMPLETE unless every targeted tenant holds the backup.
A runtime backup id, what each physical tenant reports for it, and the state aggregated over every targeted tenant — folded from the per-tenant states by the same rules a per-tenant state is folded from its partitions.