This parameter lets you split a database file into several sections (only Oracle 11g or higher). This enables the backup of the individual sections to run in parallel, which is important especially for bigfile tablespaces.
rman_sectionsize = <size> [K|M|G]
Default: 0 (no limit)
<size>: section size in KB (default), MB, or GB
See also:
RMAN Backup with an External Backup Library
Oracle documentation