With the "Backup metadata" option checked you will later be able to restore mailboxes from the backup. Otherwise – you will not
Microsoft Exchange database metadata contains information about database objects (e.g. mailboxes).
In Acronis Recovery for Microsoft Exchange the Backup metadata option is set by default.
Metadata backup adds metadata to the backup archive of Information Store. Mailboxes can only be restored when metadata is backed up.
If you uncheck this option when backing up, you will not be able to restore mailboxes from the archive.
The Backup metadata option is in Additional settings of default Backup options (Tools -> Options).