Backup piece is a physical binary file created by RMAN during a backup. They are written to a backup medium, such as disk or tape. Backup pieces contain blocks from the target
The following rules must be kept in mind while constructing a backup piece from datafiles:
i. A datafile cannot span backup sets
ii. A datafile can span backup pieces as long as it stays within one backup set
iii. Datafiles and control files can coexist in the same backup sets
iv. Archived redo log files are never in the same backup set as datafiles or control files
v. RMAN is the only tool that can operate on backup pieces; therefore, you must use the RMAN tool if you need to restore a file from an RMAN backup. You cannot manually reconstruct the database files from the backup pieces; therefore, you must use RMAN to restore files from a backup piece.