Answer

The cross-platform restore functionality is based on the capability of the Microsoft SQL Server.
As noted at the following Microsoft link:

• SQL Server 2008: http://msdn.microsoft.com/en-us/library/ms191253(v=sql.100).aspx
• SQL Server 2012: http://msdn.microsoft.com/en-us/library/ms191253.aspx

it is possible to restore an SQL legacy database backup to a different SQL Server, for example restoring a database from an SQL 2005 Server backup to an SQL 2008 Server.
This type of alternate restore is only possible with user databases, not the databases used by the SQL Server itself, such as master, msdb, etc.

It is important to note that the backups can only be restored to the same or newer version of the SQL Server. Also, as referenced in the Microsoft documents, the SQL Server backup and restore works across supported operating systems, whether they are 64-bit or 32-bit systems.

It is necessary to ensure that the Data Protection for SQL version used for the restore is the same (or newer) version then was used for the backup. For example if the backup was performed with the Data Protection 6.4 client, the restore can be performed with 6.4 or the version 7.1 Data Protection software. You cannot perform the restore with the 6.3 release.