This parameter specifies the size of the buffer used when restoring the database if a buffer size is not explicitly specified when calling the restore utility. For more information about the restore utility, refer to the Command Reference.
When restoring a database, the data is first copied from the backup media to an internal buffer. Data is then written from this buffer to the target database media when the buffer is full.
Tuning this buffer size can help improve the performance of the restore database utility as well as minimize the impact on the performance of other concurrent database operations.