The snapshot copy contains the metadata information associated with the primary copy.
If you need to create additional copies of snapshot backups, then with the backup copy operation you replicate the data of the primary copy and with the snapshot copy operation, you create a copy of the metadata associated with the primary copy.
You can associate the snapshot copy with the subclient from which you run the snapshot backup. In this case, when you run a snapshot backup, the backup operation modifies the primary snapshot copy.