Block-level backups of Exchange databases provide you with the ability to recover individual messages without restoring the entire database. When specific messages need to be recovered, only the required extents are recalled.
By default, the database is backed up in 4096 KB (4 MB) extents; however, you can change that value.
Block-level backup is supported for database that is backed up as full. Only logs are backed up when databases are backed up as incremental.