With the advent of big file tablespaces, data warehouses have the opportunity to consolidate a large number of data files into fewer, better managed data files. For backing up very large data files, RMAN provides multisection backups as a way to parallelize the backup operation within the file itself, such that sections of a file are backed up in parallel, rather than backing up on a per-file basis.
For example, a one TB data file can be sectioned into ten 100 GB backup pieces, with each section backed up in parallel, rather than the entire one TB file backed up as one file. The overall backup time for large data files can be dramatically reduced.
Oracle Database Backup and Recovery User's Guide for more information about configuring multisection backups