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 an 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 Name: ORCLPDB   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   10      Full 14693446   20/07/2019 12:06:16              NO    /u01/app/oracle/oradata/orcl/orclpdb/sysaux01.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 163     Full    276.60M    DISK        00:04:11     20/07/2019 12:10:27         BP Key: 163   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/7F43A22D9E7528CCE055FAC8EF8905E8/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6chg3m_.bkp   List of Datafiles in backup set 163   Container ID: 2, PDB Name: PDB$  SEED   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   6       Full 1441967    12/01/2019 11:41:59              NO    /u01/app/oracle/oradata/orcl/pdbseed/sysaux01.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 164     Full    503.65M    DISK        00:05:59     20/07/2019 12:12:14         BP Key: 164   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6ch8s4_.bkp   List of Datafiles in backup set 164   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   3       Full 14693445   20/07/2019 12:06:16              NO    /u01/app/oracle/oradata/orcl/sysaux01.dbf   7       Full 14693445   20/07/2019 12:06:16              NO    /u01/app/oracle/oradata/orcl/users01.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 165     Full    738.59M    DISK        00:06:22     20/07/2019 12:12:37         BP Key: 165   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6ch803_.bkp   List of Datafiles in backup set 165   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   1       Full 14693444   20/07/2019 12:06:15              NO    /u01/app/oracle/oradata/orcl/system01.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 166     Full    285.26M    DISK        00:01:08     20/07/2019 12:13:58         BP Key: 166   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/8D6E79BBBC2A140CE0531400A8C06C3B/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6cvr4x_.bkp   List of Datafiles in backup set 166   Container ID: 4, PDB Name: HOMOL   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   57      Full 14693594   20/07/2019 12:12:54              NO    /u01/app/oracle/oradata/orcl/ORCL/8D6E79BBBC2A140CE0531400A8C06C3B/datafile/o1_mf_sysaux_glh6b049_.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 167     Full    198.24M    DISK        00:01:35     20/07/2019 12:15:22         BP Key: 167   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/7F43A22D9E7528CCE055FAC8EF8905E8/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6cxvsl_.bkp   List of Datafiles in backup set 167   Container ID: 2, PDB Name: PDB$  SEED   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   5       Full 1441967    12/01/2019 11:41:59              NO    /u01/app/oracle/oradata/orcl/pdbseed/system01.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 168     Full    222.91M    DISK        00:02:53     20/07/2019 12:15:49         BP Key: 168   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/7F4405983DB33E47E055FAC8EF8905E8/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6cxov8_.bkp   List of Datafiles in backup set 168   Container ID: 3, PDB Name: ORCLPDB   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   9       Full 14693605   20/07/2019 12:13:22              NO    /u01/app/oracle/oradata/orcl/orclpdb/system01.dbf   12      Full 14693605   20/07/2019 12:13:22              NO    /u01/app/oracle/oradata/orcl/orclpdb/users01.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 169     Full    204.08M    DISK        00:02:21     20/07/2019 12:17:20         BP Key: 169   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/8D6E79BBBC2A140CE0531400A8C06C3B/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6d1dhk_.bkp   List of Datafiles in backup set 169   Container ID: 4, PDB Name: HOMOL   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   56      Full 14693679   20/07/2019 12:15:52              NO    /u01/app/oracle/oradata/orcl/ORCL/8D6E79BBBC2A140CE0531400A8C06C3B/datafile/o1_mf_system_glh69zz4_.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 170     Full    3.07M      DISK        00:02:04     20/07/2019 12:18:00         BP Key: 170   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/7F4405983DB33E47E055FAC8EF8905E8/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6d4lp2_.bkp   List of Datafiles in backup set 170   Container ID: 3, PDB Name: ORCLPDB   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   11      Full 14693696   20/07/2019 12:16:40              NO    /u01/app/oracle/oradata/orcl/orclpdb/undotbs01.dbf   60      Full 14693696   20/07/2019 12:16:40              NO    /u01/app/oracle/product/12.2.0/dbhome_1/dbs/brelele  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 171     Full    7.98M      DISK        00:00:36     20/07/2019 12:18:13         BP Key: 171   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/7F4405983DB33E47E055FAC8EF8905E8/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6d5kdw_.bkp   List of Datafiles in backup set 171   Container ID: 3, PDB Name: ORCLPDB   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   55      Full 14693724   20/07/2019 12:17:54              NO    /u01/app/oracle/oradata/orcl/ORCL/7F4405983DB33E47E055FAC8EF8905E8/datafile/o1_mf_rman_gldpgddc_.dbf   61      Full 14693724   20/07/2019 12:17:54              NO    /u01/app/oracle/product/12.2.0/dbhome_1/dbs/brelele2  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 172     Full    37.42M     DISK        00:00:34     20/07/2019 12:18:44         BP Key: 172   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/7F43A22D9E7528CCE055FAC8EF8905E8/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6d61cl_.bkp   List of Datafiles in backup set 172   Container ID: 2, PDB Name: PDB$  SEED   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   8       Full 1441967    12/01/2019 11:41:59              NO    /u01/app/oracle/oradata/orcl/pdbseed/undotbs01.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 173     Full    2.57M      DISK        00:00:31     20/07/2019 12:19:01         BP Key: 173   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/8D6E79BBBC2A140CE0531400A8C06C3B/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6d6nnb_.bkp   List of Datafiles in backup set 173   Container ID: 4, PDB Name: HOMOL   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   58      Full 14693749   20/07/2019 12:18:34              NO    /u01/app/oracle/oradata/orcl/ORCL/8D6E79BBBC2A140CE0531400A8C06C3B/datafile/o1_mf_undotbs1_glh6b04c_.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 174     Full    1.04M      DISK        00:00:32     20/07/2019 12:19:16         BP Key: 174   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/8D6E79BBBC2A140CE0531400A8C06C3B/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6d7mhm_.bkp   List of Datafiles in backup set 174   Container ID: 4, PDB Name: HOMOL   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   59      Full 14693756   20/07/2019 12:18:51              NO    /u01/app/oracle/oradata/orcl/ORCL/8D6E79BBBC2A140CE0531400A8C06C3B/datafile/o1_mf_users_glh795h1_.dbf  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 175     Full    16.57M     DISK        00:00:16     20/07/2019 12:19:21         BP Key: 175   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T120613         Piece Name: /u01/app/oracle/recovery_area/ORCL/backupset/2019_07_20/o1_mf_nnndf_TAG20190720T120613_gm6d7q9r_.bkp   List of Datafiles in backup set 175   File LV Type Ckp SCN    Ckp Time            Abs Fuz SCN Sparse Name   ---- -- ---- ---------- ------------------- ----------- ------ ----   4       Full 14694166   20/07/2019 12:19:15 14694171     NO    /u01/app/oracle/oradata/orcl/undotbs01.dbf   28      Full 14694166   20/07/2019 12:19:15              NO    /u01/app/oracle/oradata/orcl/ORCL/datafile/o1_mf_haha_gb1k1h6b_.dbf  BS Key  Size       Device Type Elapsed Time Completion Time ------- ---------- ----------- ------------ ------------------- 176     4.99M      DISK        00:00:00     20/07/2019 12:19:37         BP Key: 176   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T121936         Piece Name: /u01/app/oracle/recovery_area/ORCL/backupset/2019_07_20/o1_mf_annnn_TAG20190720T121936_gm6d89jz_.bkp    List of Archived Logs in backup set 176   Thrd Seq     Low SCN    Low Time            Next SCN   Next Time   ---- ------- ---------- ------------------- ---------- ---------   1    59      14693433   20/07/2019 12:06:09 14694370   20/07/2019 12:19:30  BS Key  Type LV Size       Device Type Elapsed Time Completion Time ------- ---- -- ---------- ----------- ------------ ------------------- 177     Full    18.09M     DISK        00:00:05     20/07/2019 12:19:54         BP Key: 177   Status: AVAILABLE  Compressed: NO  Tag: TAG20190720T121948         Piece Name: /u01/app/oracle/recovery_area/ORCL/autobackup/2019_07_20/o1_mf_s_1014121189_gm6d8tj6_.bkp   SPFILE Included: Modification time: 20/07/2019 12:18:10   SPFILE db_unique_name: ORCL   Control File Included: Ckp SCN: 14694951     Ckp time: 20/07/2019 12:19:49 

shouldn’t the rman creates one backupset for one backup and in this backupset include all backup pieces?