For an ArchiveRetrieval job, it is the checksum of the archive. Otherwise, the value is null.
The SHA256 tree hash value for the requested range of an archive. If the
Initiate a Job request for an archive specified a tree-hash aligned range, then this field returns a value.
For the specific case when the whole archive is retrieved, this value is
the same as the ArchiveSHA256TreeHash value.
This field is null in the following situations: Archive retrieval jobs
that specify a range that is not tree-hash aligned.
Archival jobs that specify a range that is equal to the whole archive
and the job status is InProgress.