How is the behaviour of backupset in RMAN?

I found this definition on WEB:


A logical grouping of backup files -- the backup pieces -- that are
created when you issue an RMAN backup command. A backup set is RMAN’s
name for a collection of files associated with a backup. A backup set
is composed of one or more backup pieces.

but I don't understand why when I make one backup of whole database the rman creates several backupsets

Here is the output about one backup that I make (I've used parallelism 4)

```
[oracle@orcl12 ~]$ rman target /

Recovery Manager: Release 12.2.0.1.0 - Production on Sat Jul 20 12:48:52 2019

Copyright (c) 1982, 2017, Oracle and/or its affiliates. All rights reserved.

connected to target database: ORCL (DBID=1525136414)

RMAN> list backup;

using target database control file instead of recovery catalog

List of Backup Sets
===================

BS Key Size Device Type Elapsed Time Completion Time
------- ---------- ----------- ------------ -------------------
161 3.92M DISK 00:00:01 20/07/2019 12:06:12
BP Key: 161 Status: AVAILABLE Compressed: NO Tag: TAG20190720T120610
Piece Name: /u01/app/oracle/recovery_area/ORCL/backupset/2019_07_20/o1_mf_annnn_TAG20190720T120610_gm6ch3t2_.bkp

List of Archived Logs in backup set 161
Thrd Seq Low SCN Low Time Next SCN Next Time
---- ------- ---------- ------------------- ---------- ---------
1 58 14688057 20/07/2019 11:55:34 14693433 20/07/2019 12:06:09

BS Key Type LV Size Device Type Elapsed Time Completion Time
------- ---- -- ---------- ----------- ------------ -------------------
162 Full 307.59M DISK 00:03:59 20/07/2019 12:10:15
BP Key: 162 Status: AVAILABLE Compressed: NO Tag: TAG20190720T120613
Piece Name: /u01/app/oracle/recovery_area/ORCL/7F4405983DB33E47E055FAC8EF8905E8/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6chcwh_.bkp
List of Datafiles in backup set 162
Container ID: 3, PDB

Also from the Database Backup and Recovery User's Guide.


The total number and size of backup sets depends mostly on an internal
RMAN algorithm. However, you can influence RMAN behavior with the
MAXSETSIZE parameter in the CONFIGURE or BACKUP command. By limiting
the size of the backup set, the parameter indirectly limits the number
of files in the set and can possibly force RMAN to create additional
backup sets. Also, you can specify BACKUP ... FILESPERSET to specify
the maximum number of files in each backup set.

StackExchange Database Administrators Q#243372, answer score: 2